P0377 High Resolution Time Reference Signal B Too Few Pulses

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have an engine position sensor (Chevrolet, GMC, Ford, Dodge, BMW, etc.). Although generic, the specific repair steps may vary by make/model.

The Powertrain Control Module (PCM) requires a timing reference signal to determine engine speed and position. This is accomplished via an engine position sensor, such as a crankshaft position sensor, camshaft position sensor, or fuel pump sensor.

The PCM uses this information to determine one or more of the following: fuel injector control, ignition timing, and misfire diagnostics. There are two basic types of engine position sensors: Hall effect and permanent magnet.

Both sensor types are triggered by a wheel or disc mounted near the sensor. Typical wheel/disc mounting locations include the engine position, injection pump, harmonic balancer, and distributor shaft. When triggered, the sensor produces a return signal that is sent back to the PCM.

    Hall effect crankshaft position sensors have three wires: reference (power), signal, and ground. The sensor receives a reference voltage (typically 5 volts) from the PCM and is triggered by a reluctor wheel. The sensor then sends a corresponding digital return signal to the PCM.
    Permanent magnet (PM) sensors generate their own alternating voltage based on their proximity to a rotating reluctor wheel. This voltage signal is sent back to the PCM where it is converted to a digital signal by an internal analog-to-digital converter.

In the codes described in this article, the “B” indicates that the problem relates to a part of the system circuit, rather than a specific symptom or component.

A P0377 code is set when the PCM detects that the engine position sensor “B” is producing a signal with too few pulses.

Code Severity and Symptoms

The severity of these codes is moderate to severe. Symptoms of a P0377 engine code may include:

    Illuminated check engine light
    Hard starting / no start
    Poor engine performance

Causes

Possible causes of this P0377 code are as follows:

    Faulty sensor
    Wiring issues
    Damaged reluctor wheel
    Contaminated oil
    Faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the engine position sensor and its associated wiring. Look for loose connections, damaged wiring, etc. If damage is found, repair as necessary, clear the code, and see if it returns. Also check that the engine does not contain any metal particles that could affect sensor operation.

Next, check for Technical Service Bulletins (TSBs) related to the issue. If none are found, proceed with step-by-step system diagnostics.

The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to the manufacturer’s diagnostic flowchart. Testing procedures also depend on the type of sensor found in your vehicle.

Check the Sensor

Hall effect sensor: Use a digital multimeter set to DC volts and a back-probe test lead to ensure the sensor is producing a signal voltage. Connect one meter lead to the sensor’s signal wire and the other to ground. Have an assistant crank the engine by turning the ignition key (do not start the engine); the voltage reading should fluctuate between 0 volts and the reference voltage. If available, you can also use an oscilloscope to visualize the signal pulses while cranking the engine.

Permanent magnet sensor: Start by checking the sensor’s internal resistance. Disconnect the sensor’s electrical connector and connect a digital multimeter set to ohms across the sensor’s terminals. The resistance should be approximately 500 to 1,200 ohms. Consult factory repair information for exact specifications. If the resistance test is OK, you’ll want to test the sensor’s performance. To do this, reconnect the sensor’s electrical connector. Then, connect a digital multimeter set to AC volts to the sensor’s electrical connector using back-probe test leads. Since the sensor produces alternating voltage, it doesn’t matter how the multimeter leads are connected. Have an assistant start the engine; you should see a fluctuating voltage reading.

One thing to remember when testing the sensor is a damaged or misaligned tone ring that will prevent the sensor from functioning correctly. If in doubt, access the tone ring and inspect it if necessary.
Check the Circuit

It’s a good idea to have the factory wiring diagram on hand to determine which wire is which while performing these tests.

Hall effect sensor:

    Check the ground side of the circuit: Connect the ohmmeter (ignition off) between the sensor connector’s ground terminal and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit, which must be located and repaired. If the meter reads a numerical value, there is continuity.
    Check the reference voltage side of the circuit: Ignition on, use a digital multimeter set to volts to check for the PCM reference (usually 5 volts) at one of the sensor’s three terminals. If no reference signal is present, connect the ohmmeter (ignition off) between the reference voltage terminal on the sensor and the reference voltage terminal on the PCM. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity. Next, you’ll want to verify that there are 5 volts coming from the PCM at the reference voltage terminal. If there is no 5-volt reference from the PCM, the PCM is likely faulty.
    Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check for continuity between the engine position sensor’s output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

Permanent magnet sensor:

    Check the ground side of the circuit: Connect the ohmmeter (ignition off) between the SENSOR connector’s ground terminal and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit that will need to be located and repaired. If the meter reads a numerical value, there is continuity.
    Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check for continuity between the engine position sensor’s output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

P0378 High Resolution Synchronization Reference Signal B Intermittent/Irregular Pulses

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have an engine position sensor (Chevrolet, GMC, Ford, Dodge, BMW, etc.). Although generic, the exact repair steps may vary by make/model.

The Powertrain Control Module (PCM) requires a timing reference signal to determine engine speed and position. This is accomplished by means of an engine position sensor, such as a crankshaft position sensor, camshaft position sensor, or fuel pump sensor.

The PCM uses this information to determine one or more of the following: fuel injector control, ignition timing, and misfire diagnostics. There are two basic types of engine position sensors: Hall effect and permanent magnet.

Both types of sensors are triggered by a wheel or disc mounted near the sensor. Typical wheel/disc mounting locations include the engine position, injection pump, harmonic balancer, and distributor shaft. When triggered, the sensor produces a return signal that is sent back to the PCM.

    Hall effect crankshaft position sensors have three wires: reference (power), signal, and ground. The sensor receives a reference voltage (usually 5 volts) from the PCM and is triggered by a reluctor wheel. The sensor then sends a corresponding digital return signal to the PCM.
    Permanent magnet (PM) sensors produce their own alternating voltage based on their proximity to a rotating reluctor wheel. This voltage signal is sent back to the PCM where it is converted to a digital signal by an internal analog-to-digital converter.

In the codes described in this article, the “B” indicates that the problem is with a part of the system circuit, rather than a specific symptom or component.

A P0378 code is set when the PCM detects that the engine position sensor “B” is producing a signal with intermittent/erratic pulses.

Code Severity and Symptoms

The severity of these codes is moderate to severe. Symptoms of a P0378 engine code may include:

    Check Engine Light illuminated
    Hard starting/no start
    Poor engine performance

Causes

Possible causes of this P0378 code include:

    Faulty sensor
    Wiring issues
    Damaged reluctor wheel
    Contaminated oil
    Faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the engine position sensor and its associated wiring. Look for loose connections, damaged wiring, etc. If damage is found, repair as necessary, clear the code, and see if it returns. Also check that the engine does not contain any metal particles that could affect the sensor’s operation.

Next, check for Technical Service Bulletins (TSBs) related to the issue. If none are found, proceed with step-by-step system diagnostics.

The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to the manufacturer’s diagnostic flowchart. Testing procedures also depend on the type of sensor found in your vehicle.

Check the Sensor

Hall effect sensor: Use a digital multimeter set to DC volts and a back-probe test lead to ensure the sensor is producing a signal voltage. Connect one meter lead to the sensor’s signal wire and the other to ground. Have an assistant crank the engine by turning the ignition key (do not start the engine); the voltage reading should fluctuate between 0 volts and the reference voltage. If available, you can also use an oscilloscope to visualize the signal pulses while cranking the engine.

Permanent magnet sensor: Start by checking the sensor’s internal resistance. Disconnect the sensor’s electrical connector and connect a digital multimeter set to ohms to each of the sensor’s terminals. The resistance should be approximately 500 to 1,200 ohms. Consult factory repair information for exact specifications. If the resistance test is OK, you will want to test the sensor’s performance. To do this, reconnect the sensor’s electrical connector. Then, connect a digital multimeter set to AC volts to the sensor’s electrical connector using back-probe test leads. Since the sensor produces an alternating voltage, it does not matter how the multimeter leads are connected. Have an assistant start the engine; you should see a fluctuating voltage reading.

One thing to remember when testing the sensor is a damaged or misaligned tone ring that will prevent the sensor from functioning properly. If in doubt, access the tone ring and inspect it if necessary.
Check the Circuit

It’s a good idea to have the factory wiring diagram on hand to determine which wire is which while performing these tests.

Hall effect sensor:

    Check the ground side of the circuit: Connect the ohmmeter (ignition off) between the ground terminal of the sensor connector and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit, which must be located and repaired. If the meter reads a numerical value, there is continuity.
    Check the reference voltage side of the circuit: Ignition on, use a digital multimeter set to volts to check for the PCM reference (usually 5 volts) at one of the three sensor terminals. If no reference signal is present, connect the ohmmeter (ignition off) between the reference voltage terminal on the sensor and the reference voltage terminal on the PCM. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity. Next, you will want to verify that there are 5 volts coming from the PCM at the reference voltage terminal. If there is no 5-volt reference from the PCM, the PCM is likely faulty.
    Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check for continuity between the engine position sensor output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

Permanent magnet sensor:

    Check the ground side of the circuit: Connect the ohmmeter (ignition off) between the ground terminal of the SENSOR connector and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit that will need to be located and repaired. If the meter reads a numerical value, there is continuity.
    Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check for continuity between the engine position sensor output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

P0379 High Resolution Synchronization Reference Signal B No Pulse

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have an engine position sensor (Chevrolet, GMC, Ford, Dodge, BMW, etc.). Although generic, the exact repair steps may vary by make/model.

The Powertrain Control Module (PCM) requires a timing reference signal to determine engine speed and position. This is accomplished by means of an engine position sensor, such as a crankshaft position sensor, camshaft position sensor, or fuel pump sensor.

The PCM uses this information to determine one or more of the following: fuel injector control, ignition timing, and misfire diagnostics. There are two basic types of engine position sensors: Hall effect and permanent magnet.

Both types of sensors are triggered by a wheel or disc mounted near the sensor. Typical wheel/disc mounting locations include the engine position, injection pump, harmonic balancer, and distributor shaft. When triggered, the sensor produces a return signal that is sent back to the PCM.

Hall effect crankshaft position sensors have three wires: reference (power), signal, and ground. The sensor receives a reference voltage (usually 5 volts) from the PCM and is triggered by a reluctor wheel. The sensor then sends a corresponding digital return signal to the PCM.
Permanent magnet (PM) sensors produce their own alternating voltage based on their proximity to a rotating reluctor wheel. This voltage signal is sent back to the PCM where it is converted to a digital signal by an internal analog-to-digital converter.

In the codes described in this article, the “B” indicates that the problem pertains to a part of the system circuit, rather than a specific symptom or component.

A P0379 code is set when the PCM detects that the engine position sensor “B” signal has no pulses.

Code Severity and Symptoms

The severity of these codes is moderate to severe. Symptoms of a P0379 engine code may include:

Illuminated check engine light
Hard starting/no start
Poor engine performance

Causes

Possible causes of this P0379 code include:

Faulty sensor
Wiring issues
Damaged reluctor wheel
Contaminated oil
Faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the engine position sensor and its corresponding wiring. Look for loose connections, damaged wiring, etc. If damaged, repair as necessary, clear the code, and see if it returns. Also check that the engine does not contain any metal particles that could affect sensor operation.

Next, check for technical service bulletins (TSBs) regarding the issue. If none are found, proceed with step-by-step diagnosis of the system.

The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to the manufacturer’s diagnostic flowchart. Testing procedures also depend on the type of sensor found in your vehicle.

Check the Sensor

Hall effect sensor: Use a digital multimeter set to DC volts and a back-probe test lead to ensure the sensor is producing a signal voltage. Connect one meter lead to the sensor’s signal wire and the other to ground. Have an assistant crank the engine by turning the ignition key (do not start the engine); the voltage reading should fluctuate between 0 volts and the reference voltage. If available, you can also use an oscilloscope to visualize the signal pulses while cranking the engine.

Permanent magnet sensor: Start by checking the sensor’s internal resistance. Disconnect the sensor’s electrical connector and connect a digital multimeter set to ohms to each of the sensor’s terminals. The resistance should be approximately 500 to 1,200 ohms. Consult factory repair information for exact specifications. If the resistance test is OK, you will want to test the sensor’s performance. To do this, reconnect the sensor’s electrical connector. Then, connect a digital multimeter set to AC volts to the sensor’s electrical connector using back-probe test leads. Since the sensor produces alternating voltage, it does not matter how the multimeter leads are connected. Have an assistant start the engine; you should see a fluctuating voltage reading.

One thing to remember when testing the sensor is a damaged or misaligned tone ring that will prevent the sensor from functioning properly. If in doubt, access the tone ring and inspect it if necessary.
Check the Circuit

It’s a good idea to have the factory wiring diagram on hand to determine which wire is which while performing these tests.

Hall effect sensor:

Check the ground side of the circuit: Connect the meter to ohms (ignition off) between the sensor connector’s ground terminal and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit, which must be located and repaired. If the meter reads a numerical value, there is continuity.
Check the reference voltage side of the circuit: Ignition on, use a digital multimeter set to volts to check the PCM’s reference (usually 5 volts) at one of the sensor’s three terminals. If no reference signal is present, connect the meter to ohms (ignition off) between the reference voltage terminal on the sensor and the reference voltage terminal on the PCM. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity. Next, you will want to verify that there are 5 volts coming from the PCM at the reference voltage terminal. If there is no 5-volt reference from the PCM, the PCM is likely faulty.
Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check continuity between the engine position sensor’s output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

Permanent magnet sensor:

Check the ground side of the circuit: Connect the meter to ohms (ignition off) between the SENSOR connector’s ground terminal and ground. If the meter reads out of limits (OL), there is an open circuit on the ground side of the circuit that will need to be located and repaired. If the meter reads a numerical value, there is continuity.
Check the return signal side of the circuit: Ignition off, use a digital multimeter set to ohms to check continuity between the engine position sensor’s output terminal and the signal voltage terminal on the PCM. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired.

P037A, P037B, P037C ISO/SAE Reserved

P00C0 – P00FF ISO/SAE Reserved

P037D Glow Plug Detection Circuit

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have glow plugs (diesel engine vehicles). Vehicle makes may include but are not limited to Ford, Dodge, Mazda, VW, Ram, GMC, Chevy, etc. Although generic, specific repair steps may vary by make/model/engine. As a side note, this code appears more common on Ford vehicles.

Glow plugs and their associated wiring harnesses and circuits are part of a system that is responsible for generating heat in the combustion chamber prior to cold starts.

Essentially, the glow plug is similar to an element on a stove. These are incorporated into diesel engines because diesel engines do not use spark plugs to ignite the fuel/air mixture. Instead, they use compression to generate enough heat to ignite the mixture. For this reason, diesel engines need the help of glow plugs during cold starts.

The ECM sets P037D and related codes when it monitors a condition outside of a specific range in the glow plug circuit. Most of the time, I would say this would be an electrical issue, but some mechanical problems could affect your glow plug circuit on certain makes and models. P037D Glow Plug Sense Circuit code is set when the ECM monitors one or more values outside of a specific range.

An example of a glow plug:

NOTE: If other dashboard warning lights are currently on (e.g., traction control, ABS, etc.), this could be a sign of another potentially more significant issue. In this case, you should take your vehicle to a reputable shop where it can be connected to a proper scan tool to avoid causing unnecessary damage.

This fault code is closely related to P037E and P037F.
How severe is this DTC?

Generally speaking, the severity of this code would be moderate, but depending on the scenario, it can be severe. For example, if you reside in moderate to extreme cold conditions, repetitive cold starts with faulty glow plugs will eventually unnecessarily damage internal components of your engine.
What are some of the symptoms of the code?

Symptoms of a P037D engine code may include:

Hard starting in the morning or when it’s cold
Abnormal engine noises during startup
Poor performance
Engine misfire
Poor fuel consumption

What are some common causes of the code?

Causes

of this code may include:

Broken or damaged wiring harness
Blown/defective fuse link
Faulty glow plug
ECM problem
Pin/connector issue (e.g., corrosion, overheating, etc.)

What are the troubleshooting steps?

Make sure to check the Technical Service Bulletins (TSBs) for your vehicle. Accessing a known fix can save you time and money during diagnosis.
Tools

Whenever working with electrical systems, some of the basic tools that are a good idea are:

OBD code reader
Multimeter
Basic socket set
Ratchet and wrench sets
Basic screwdriver set
Rags/shop towels
Battery terminal cleaner
Service manual

Safety Tips

Let the engine cool down
Chock the wheels
Wear PPE (Personal Protective Equipment)

Basic Step #1

The first thing I would do in this situation is open the hood and smell for any kind of irregular burning scent. If present, this could be the trail to your problem. Most of the time, if there are excessive burning smells, it means something is overheating. Carefully follow the smell; if you see burnt wire coatings or melting plastic around fuse boxes, fuse links, etc., this will need to be addressed first.

NOTE: Visually inspect all ground straps and look for corroded or loose ground connections.
Basic Step #2

Locate and follow the glow plug circuit harness. These harnesses are exposed to extreme heat that can damage the looms that are there to protect your wires. In particular, keep an eye out for spots on the harness that may rub against the engine or other components. Repair any damaged wires or looms.
Basic Tip #3

If possible, unplug your glow plug harness from the plugs. In some cases, you may also disconnect it from the other side of the harness and remove it completely from the vehicle as an assembly. If this is the case, you can use your multimeter to check for continuity in the individual wires of the circuit. This would rule out any kind of physical problem in that harness. In some vehicles, this may not be possible. If not, skip this step.

NOTE: Make sure to disconnect the battery before performing any electrical repairs.
Basic Step #4

Test your circuits. Consult the manufacturer for specific desired electrical values. Using your multimeter, you can perform many tests to check the integrity of the circuits involved.
Basic Step #5

Test your glow plugs. Unplug the harness from the plugs. Using your multimeter set to Voltage, you will attach one end to the positive battery terminal and using the other end, you will touch the tip of each plug. The values should be the same as the battery voltage; if they are not, this would indicate a problem in the plug itself. This may vary depending on your specific vehicle’s make and model.

P037E Low Glow Plug Detection Circuit

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have glow plugs (diesel engine vehicles). Vehicle makes may include, but are not limited to, Ford, Dodge, Mazda, VW, Ram, GMC, Chevy, etc. Although generic, specific repair steps may vary by make/model/engine. As a side note, this code seems more common on Ford vehicles.

Glow plugs and their associated wiring harnesses and circuits are part of a system that is responsible for generating heat in the combustion chamber prior to cold starts.

Essentially, the glow plug is similar to an element on a stove. These are incorporated into diesel engines because diesel engines do not use spark plugs to ignite the fuel/air mixture. Instead, they use compression to generate enough heat to ignite the mixture. For this reason, diesel engines need the help of glow plugs during cold starts.

The ECM sets P037E and related codes when it monitors a condition outside of a specific range in the glow plug circuit. Most of the time, I would say this would be an electrical issue, but some mechanical problems could affect your glow plug circuit on certain makes and models. P037E Glow Plug Sense Circuit Low code is set when the ECM monitors one or more low electrical values in the circuit.

An example glow plug:

NOTE: If other dashboard warning lights are currently on (e.g., traction control, ABS, etc.), this could be a sign of another potentially more significant issue. In this case, you should take your vehicle to your reputable shop where it can be connected to a proper scan tool to avoid causing unnecessary damage.

This fault code is closely related to P037D and P037F.
How severe is this DTC?

Generally speaking, the severity of this code would be moderate, but depending on the scenario, it can be severe. For example, if you reside in moderate to extreme cold conditions, repetitive cold starts with faulty glow plugs will eventually unnecessarily damage internal components of your engine.
What are some of the symptoms of the code?

Symptoms of a P037E engine code may include:

Hard starting in the morning or when it’s cold
Abnormal engine noises during startup
Poor performance
Engine misfire
Poor fuel consumption

What are some of the common causes of the code?

Causes

of this code may include:

Broken or damaged wiring harness
Blown/faulty fuse link
Faulty glow plug
ECM problem
Pin/connector issue (e.g., corrosion, overheating, etc.)

What are the troubleshooting steps?

Be sure to check the Technical Service Bulletins (TSBs) for your vehicle. Accessing a known fix can save you time and money during diagnosis.
Tools

Whenever working with electrical systems, some of the basic tools that are a good idea are:

OBD code reader
Multimeter
Basic socket set
Ratchet and wrench sets
Basic screwdriver set
Rags/shop towels
Battery terminal cleaner
Service manual

Safety Tips

Let the engine cool down
Chock the wheels
Wear PPE (Personal Protective Equipment)

Basic Step #1

The first thing I would do in this situation is open the hood and smell for any kind of irregular burning scent. If present, this could be the trail to your problem. Most of the time, if there are excessive burning smells, it means something is overheating. Carefully follow the smell; if you see burnt wire coatings or melting plastic around fuse boxes, fuse links, etc., this will need to be addressed first.

NOTE: Visually inspect all ground straps and look for corroded or loose ground connections.
Basic Step #2

Locate and follow the glow plug circuit harness. These harnesses are exposed to extreme heat which can damage the looms that are there to protect your wires. In particular, keep an eye out for spots on the harness that may rub against the engine or other components. Repair any damaged wires or looms.
Basic Tip #3

If possible, unplug your glow plug harness from the plugs. In some cases, you might also disconnect it from the other side of the harness and remove it completely from the vehicle as an assembly. If that’s the case, you can use your multimeter to check for continuity in the individual wires of the circuit. This would rule out any kind of physical problem in that harness. On some vehicles, this may not be possible. If not, skip this step.

NOTE: Be sure to disconnect the battery before performing any electrical repairs.
Basic Step #4

Test your circuits. Consult the manufacturer for specific desired electrical values. Using your multimeter, you can perform many tests to check the integrity of the involved circuits.
Basic Step #5

Test your glow plugs. Disconnect the harness from the plugs. Using your multimeter set to Voltage, you will attach one end to the positive battery terminal and using the other end, you will touch the tip of each plug. The values should be the same as the battery voltage; if they are not, this would indicate a problem in the plug itself. This can vary depending on the make and model of your particular vehicle, so ALWAYS consult the manufacturer’s service information FIRST.

P037F High Glow Plug Detection Circuit

What Does It Mean?

The diagnostic trouble code (DTC) P037F is a generic powertrain code that applies to OBD-II equipped vehicles with glow plugs (diesel engine vehicles).

Affected brands include, among others: Ford, Dodge, Mazda, VW, Ram, GMC, and Chevy. Although generic, repair steps may vary depending on the vehicle’s make, model, and engine. This code appears to be more common on Ford vehicles.

Glow plugs and their circuits are responsible for generating heat in the combustion chamber before cold starts. They function similarly to a stove element and are essential because diesel engines use compression rather than spark plugs to ignite the fuel/air mixture.

The ECM (engine control module) sets the P037F code when it detects an abnormal electrical value in the glow plug circuit. Most of the time, it is an electrical issue, although some mechanical problems can also influence this circuit.

This code is closely related to codes P037D and P037E.

Severity of Code P037F

The severity of this code is moderate, but it can be severe in certain situations. For example, if you live in an area with very cold temperatures, repeated cold starts with faulty glow plugs can cause internal engine damage.

Symptoms of Code P037F

Here are the signs that may indicate the presence of this code:

  • Difficulty starting in cold weather
  • Abnormal engine noises during startup
  • Poor engine performance
  • Engine misfires
  • Increased fuel consumption

Possible Causes of Code P037F

Several causes can be at the origin of this problem:

  • Broken or damaged wiring harness
  • Blown or defective fuse link
  • Faulty glow plug
  • Problem with the ECM
  • Damaged connectors or pins (corrosion, overheating, etc.)

Troubleshooting Steps

1. Check Technical Service Bulletins (TSBs)

Before any diagnosis, consult your vehicle’s technical service bulletins. A known fix could save you unnecessary time and expense.

2. Required Tools

To work on electrical circuits, it is recommended to have the following tools:

  • OBD code reader
  • Multimeter
  • Socket and ratchet set
  • Screwdriver set
  • Shop rags/towels
  • Battery terminal cleaner
  • Vehicle service manual

3. Safety Tips

  • Allow the engine to cool before any intervention
  • Block the wheels to prevent any unintended movement
  • Wear personal protective equipment (PPE)

4. Diagnosis and Repair

Step 1: Visual Inspection

  • Open the hood and check for any burning smell.
  • Look for melted wires or damaged fuse boxes.
  • Inspect ground connections (corrosion, connection faults).

Step 2: Wiring Harness Inspection

  • Follow the glow plug harness and look for damage.
  • Check areas where the harness might rub against the engine or other components.

Step 3: Wire Continuity Check

  • If possible, disconnect the glow plug harness and test continuity with a multimeter.
  • If this is not possible on your vehicle model, proceed to the next step.

Step 4: Electrical Circuit Testing

  • Consult manufacturer specifications for expected voltage values.
  • Use a multimeter to test the involved circuits.

Step 5: Glow Plug Testing

  • Disconnect the glow plug harness.
  • With a multimeter in voltage mode, place one probe on the battery’s positive terminal and the other on the tip of each glow plug.
  • The voltage should be equal to the battery voltage. A different value indicates a problem with the glow plug.
  • Always consult manufacturer information before performing this test.

Conclusion

The P037F code indicates a problem with the glow plug circuit and can cause starting difficulties, performance losses, and excessive fuel consumption. A thorough diagnosis following the steps above can help you resolve the issue and avoid damaging your engine.

P0380 DTC Glow Plug / Heater Circuit “A” Malfunction

📌 What does the P0380 code mean?

The P0380 code is a generic OBD-II code related to the glow plug system in diesel engines. It indicates that the PCM (powertrain control module) has detected an electrical fault in the circuit “A” of the glow plug system, often due to a faulty glow plug or a wiring issue.

⚠️ Do not confuse with the P0382 code, which concerns the circuit “B” of the glow plug system.

⚠️ Common symptoms:

  • Engine light (MIL) on

  • Glow plug indicator stays on abnormally long or continuously

  • Hard starting, especially in cold weather

  • Possible white smoke at startup

🔍 Possible causes:

  • Glow plug burned out or worn

  • Fuse blown

  • Glow plug relay faulty

  • Glow plug control module defective

  • Wiring or connectors damaged (short circuit, break, corrosion)

🛠️ Diagnostic steps and possible solutions:

  1. Check the fuses related to the glow plugs, replace if necessary.

  2. Test the glow plug relay: if it’s faulty, replace it.

  3. Visually inspect the connectors, wiring, and glow plugs for signs of corrosion, broken wires, or loose connectors.

  4. Measure the resistance of each glow plug using a multimeter (DVOM). A functioning glow plug typically shows low resistance (often < 1 ohm).

  5. Check the supply voltage at the glow plug terminals when the ignition is on.

  6. Practical test: remove the glow plug and briefly supply it with 12V (quick test with caution) → if it glows red quickly, it’s working.

  7. Consult the technical service bulletins (TSB) for your vehicle: some models have known issues.

🧰 Practical tip:

Always replace glow plugs in pairs or as a full set to ensure uniform operation.

📚 Glow plug related codes:

  • P0381 to P0384: various circuit failures

  • P0670 to P0682: specific faults per cylinder

  • P068X: issues with glow plug relays or modules

P0381 Glow Plug/Heater Indicator Circuit Malfunction

What Does It Mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles manufactured after 1996 (Volkswagen, Audi, Isuzu, Dodge, Ford, etc.). Although generic, specific repair steps may vary by make/model.

When I encounter a stored P0381 code, I know the Powertrain Control Module (PCM) has detected a malfunction in the glow plug heater indicator circuit. This code is used exclusively in diesel engine vehicles. Diesel engine glow plugs are used to increase cylinder temperature in diesel engines, especially during cold start conditions.

High compression is used to ignite fuel in diesel engines. Since compression ratios are so extreme, diesel engine blocks are typically denser and heavier than gasoline engine blocks. This can result in lower cylinder temperatures that make diesel engines harder to start, particularly in cold weather. Since diesel engines achieve combustion without spark plugs, cylinder temperature is crucial for cold start efficiency.

Cylinder temperature increases when voltage is applied to glow plugs and they literally begin to glow. Most OBD-II equipped vehicles use one glow plug per cylinder, though some applications use fewer plugs strategically placed in the intake manifold or cylinder head.

Glow plug heater voltage is normally controlled by the PCM, but some vehicles also feature a standalone glow plug heater controller. Others have a glow plug heater controller integrated into the PCM. The glow plug controller uses input signals from the engine coolant temperature sensor and ambient air temperature sensor to calculate glow plug activation strategy. When the controller recognizes conditions warrant glow plug activation, an output signal is sent to a glow plug timer relay that allows battery voltage to reach the glow plugs. Since glow plugs are grounded through the engine block (or cylinder head), they become very hot and begin to glow when battery voltage is applied. Glow plug operation only occurs when engine temperature is below programmed temperature.

The glow plug heater control circuit includes the glow plug indicator. The glow plug indicator is a warning light integrated into the dashboard that illuminates when glow plugs/heaters are operating. During cold start conditions, the glow plug light typically comes on after the ignition switch is turned to ON position, and goes out before starting the engine. This brief period allows energized glow plugs to heat the cylinders before attempting to start the engine. The glow plug indicator is designed to alert the driver that glow plugs are functioning.

If the PCM detects a problem in the circuit controlling the glow plug/heater indicator, a P0381 will be stored and a malfunction indicator lamp may illuminate.
Severity and Symptoms

Glow plug operation will likely be disabled if a P0381 code is stored, so it should be considered a serious condition.

Symptoms of a P0381 code may include:

  • Delayed engine starting, especially during extremely cold weather
  • Excessive exhaust smoke
  • No glow plug indicator
  • Constant glow plug indicator illumination

Possible causes of this engine code include:

  • Faulty glow plug indicator lamp bulb
  • Defective glow plug/heater relay
  • Malfunctioning glow plug/heater controller
  • Open or shorted glow plug/heater indicator circuit
  • Faulty instrument cluster circuit board

Diagnostic and Repair Procedures

A good starting point is always checking Technical Service Bulletins (TSBs) for your specific vehicle. Your problem may be a known issue with a known fix published by the manufacturer and can save you time and money when diagnosing.

You’ll need a scan tool, digital volt/ohmmeter (DVOM), and reliable vehicle information source (like All Data DIY) to diagnose a P0381 code.

I typically begin my diagnosis with a visual inspection of all wiring and connectors related to the system. In this case, I would also test all fuses and fuse links related to glow plugs/heaters, gauges, and indicators.

I would continue by connecting the scan tool to the vehicle’s diagnostic connector to retrieve stored codes and freeze frame data. I would write down this information. If this code proves intermittent, it may prove helpful. Then I would clear the codes and operate the glow plug/heater to see if it immediately resets. If P0381 resets, consult your vehicle information source for system relay design specifications. Use the DVOM to test system circuits and relays. Perform necessary repairs and retest the system.

If the glow plug heater indicator doesn’t illuminate, suspect an open circuit, faulty relay, or bad glow plug timer controller.

If the glow plug heater light stays on, suspect a short circuit (to power) or faulty instrument cluster circuit board.

If the glow plug indicator appears to function normally and P0381 continues to reset, suspect a faulty glow plug or open glow plug/heater circuit.

Additional Diagnostic Notes:

A simple method for testing glow plugs is to remove them from the engine, ground them, and apply 12 volts. If the glow plug begins to glow, it’s working properly. If not, replace the glow plug

P0382 DTC Glow Plug / Heater Circuit “B” Malfunction

What does it mean?

This code is a generic powertrain group code. It is considered generic because it applies to all makes and models of vehicles (1996 and newer), although specific repair steps may vary slightly by model.

The description for GM vehicles is slightly different: glow plug performance conditions.

The glow plug operates when starting a cold diesel engine (the Powertrain Control Module PCM uses the coolant temperature when the ignition is turned on to determine this). The glow plug heats up for a short period to increase the cylinder temperature, which allows diesel fuel to ignite more easily. If there is an open in the glow plug or circuit, this DTC will be set.

On some diesel engines, the PCM will activate the glow plugs for a period after the engine is running to reduce white smoke and engine noise.

A typical diesel engine glow plug:

Basically, code P0382 means that the PCM has detected a fault in the glow plug/heater circuit “B”.

Note: This DTC is very similar to P0380, which refers to circuit “A”. If you have multiple DTC codes, correct them in the order they appear.

Symptoms

Symptoms of a P0382 trouble code may include:

Malfunction Indicator Lamp (MIL) illumination
Glow plug/Wait To Start indicator stays on longer than normal (may be continuously lit)
Hard starting condition, especially in cold weather

Potential Causes

Causes

Potential causes of this DTC may include:

Fault in glow plug wiring (open, short to ground, etc.)
Faulty glow plug
Blown fuse
Faulty glow plug relay
Faulty glow plug module

Diagnostic Steps

and Possible Solutions

If you have a GM truck or really any vehicle for that matter, check for known issues such as TSBs (Technical Service Bulletins) related to this code.
Check associated fuses, replace them if blown. Test the glow plug relay if possible.
Visually inspect glow plugs, wiring, and connectors for corrosion, bent/loose wiring pins, loose screws/nuts on wiring connections, burnt appearance. Repair as necessary.
Check the resistance of the harness connectors using a digital volt ohmmeter (DVOM). Compare to manufacturer specifications.
Disconnect the glow plug wires, measure resistance using the DVOM, compare to specifications.
Use a DVOM to check that the glow plug wiring connector is powered and grounded.
If replacing a glow plug, be sure to start it in the threads by hand, just as you would with a spark plug.
If you really want to test the glow plug, you can always remove them, apply 12V to the terminal and ground the body for 2-3 seconds. If it glows red, it’s good; if it’s dull red or not red, it’s no good.
If you have access to an advanced scan tool, there may be functions you can use on it related to the glow plug wiring circuit.

Other DTCs related to glow plugs: P0380, P0381, P0383, P0384, P0670, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P068.