P0669 Internal Temperature Sensor Circuit PCM / ECM / TCM High

What does it mean?

This is a generic powertrain diagnostic trouble code (DTC) that applies to many OBD-II vehicles (1996-newer). This may include, but is not limited to, vehicles from Mazda, Honda, Dodge, Ford, BMW, VW, etc. Although generic, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.

OBD-II trouble codes P0666, P0667, P0668, and P0669 are associated with the internal temperature sensor circuit of different modules. This circuit includes the Powertrain Control Module (PCM), Engine Control Module (ECM), and/or Transmission Control Module (TCM).

The purpose of the PCM/ECM/TCM internal temperature sensor circuit is to monitor the temperature of various sensors housed within the control modules. Malfunctions are identified by the control module’s self-test failure process. On some automobiles, all three modules are integrated into a consolidated unit, typically referred to as the PCM.

When the PCM, ECM, or TCM detects an electrical signal higher than expected (outside the normal operating range) in the internal temperature sensor circuit, code P0669 will be set, and the Check Engine light or Transmission Warning light will illuminate.

An example of a Transmission Control Module TCM:

P0669 TCM
How severe is this DTC?

The severity of this code can vary greatly from a simple Check Engine light or Transmission Warning light on a vehicle that starts and runs, to an automobile that may stall or not start at all. The code can be serious depending on the specific nature of the problem.
What are some of the symptoms of the code?

Symptoms of a P0669 trouble code may include:

Engine does not start
Engine may stall
Incorrect shifting
Transmission Warning light illuminated
Check Engine light illuminated

What are some of the common causes of the code?

Causes

of this P0669 code may include:

Corroded or damaged connector
Loose or faulty control module ground strap
Faulty or damaged wiring
Faulty PCM, ECM, or TCM

What are the troubleshooting steps for P0669?

The first step in the troubleshooting process for any malfunction is to search for Technical Service Bulletins (TSBs) for the specific vehicle by year, model, and powertrain. In some circumstances, this can save a significant amount of time in the long run by pointing you in the right direction.

The second step is to locate all control modules within this circuit and perform a thorough visual inspection of the associated wiring, looking for obvious defects such as chafing, rubbing, bare wires, or burn marks. This process should also include ground straps and wires. Next, check the connectors for security, corrosion, and damaged pins. This process should include the PCM, ECM, and TCM depending on the specific vehicle and control module configuration. Vehicle-specific technical data will help you locate components and the control module configuration.
Advanced Steps

Advanced steps become very vehicle-specific and require appropriate advanced equipment to perform accurately. These procedures require a digital multimeter and vehicle-specific technical references. The specific technical data will include troubleshooting charts and the proper sequence to follow to help you make an accurate diagnosis.
Voltage Checks

Specific troubleshooting guidelines must be referenced to determine the required voltage ranges for the different control modules. These references will include pin numbers and voltage requirements associated with the PCM/ECM/TCM internal temperature sensor/circuit. Most, but not all, control modules require a reference voltage of approximately 9 volts. Voltage requirements vary depending on the specific year and model of the vehicle.

If this process identifies the absence of a power source or ground, continuity tests may be required to check the integrity of the wiring, connectors, and other components. Continuity tests should always be performed with the power disconnected from the circuit, and normal readings for wiring and connections should be 0 ohms of resistance. Resistance or lack of continuity is an indication of faulty wiring that is open or shorted and must be repaired or replaced. A continuity test between the different control modules and the chassis will confirm the serviceability of the ground straps and wires. The presence of resistance indicates a loose connection or possible corrosion.
What are the common repairs for this code?

Clean corrosion from connectors
Repair or replace faulty wiring
Repair or replace faulty ground straps
Flashing or replacing the PCM, ECM, or TCM

Misdiagnosis leading to the replacement of the PCM, ECM, or TCM is often done in error when faulty wiring or poor connections are the cause of this code. Furthermore, on vehicles equipped with two or more control modules, the wrong module may be replaced by mistake.

I hope the information contained in this article has helped point you in the right direction to correct the issue with your PCM/ECM/TCM internal temperature sensor/circuit fault code. This article is strictly informational, and the specific technical data and service bulletins for your vehicle should always take priority.

P066A – Cylinder 1 Glow Plug Circuit Low

The generic OBD-II code P066A indicates that the Powertrain Control Module (PCM) has detected an abnormally low voltage in the glow plug control circuit for cylinder number 1. This code applies to many vehicles equipped with diesel engines (VW, Porsche, Ford, Toyota, GM, Chevrolet, Jeep, Chrysler, Dodge, Ram, etc.).

Role of Glow Plugs
Unlike gasoline engines, diesel engines use high compression to ignite the fuel. Glow plugs, screwed into each cylinder, rapidly increase the combustion chamber temperature during startup (especially when cold). Their activation is controlled by a timer (or controller/module) and/or the PCM. Correct voltage causes them to glow.

Severity and Symptoms
P066A is an urgent code to address, as it typically causes:

Hard or delayed starting (especially in cold weather)
Poor engine performance (misfires, power loss)
Excessive black smoke from the exhaust
Decreased fuel efficiency
Possible misfire codes
Illumination of the Malfunction Indicator Lamp (MIL)

Probable Causes

Faulty glow plug (cylinder #1) – A common cause even for a circuit code.
Electrical problem: Open or short circuit (damaged wires, corrosion) in the control wiring for cylinder #1.
Faulty connector: Poor contact, oxidation, or damage on the glow plug #1 connector.
Control module failure: Defective glow plug timer/controller or PCM.

Diagnostic Procedure (Key Steps)
Accurate diagnosis requires:

OBD-II Scanner: Retrieve codes and freeze frame data. Check for vehicle-specific Technical Service Bulletins (TSBs).
Technical documentation: Wiring diagrams, locations (cylinder #1!), glow plug resistance specifications.
Digital Multimeter (DVOM): Voltage and continuity measurements.
Visual inspection: Carefully check wiring, connectors, and fuses associated with the glow plug circuit.

Verification Steps:

Confirm cylinder #1: Essential. Consult the vehicle documentation for its exact location.
Glow plug test (practical method):
- Disconnect and remove the suspect glow plug (cyl. 1).
- Apply battery voltage directly (+ and ground). A healthy glow plug should glow red quickly and evenly. Caution: Risk of burns and fire.
- Check its resistance with the DVOM (compare to manufacturer specifications).
Control circuit test:
- Reconnect the glow plug (if good).
- Use the scanner to activate the glow plug control via the PCM/controller.
- Measure the voltage at the glow plug connector with the DVOM. Normal battery voltage should be present.
- If voltage is absent or low: Check the voltage at the output of the glow plug controller or PCM.
- Thoroughly test all related fuses and relays (under load if possible).
Circuit verification:
- If the controller/PCM sends the voltage correctly but it doesn’t reach the glow plug, look for a break (open circuit) or a short circuit in the wiring or connector.
- Check the quality of the circuit ground.

Important Point: Do not overlook the glow plug itself as a possible cause, even for a circuit code. A faulty glow plug can disrupt the control circuit.

P066B Cylinder 1 Glow Plug Circuit High

🚗 Error Code P066B – Glow Plug Cylinder #1

❓ What Does It Mean?

This diagnostic trouble code (DTC) is generic and applies to many OBD-II vehicles (Jeep, Chrysler, BMW, Toyota, Volkswagen, Dodge, Ram, Ford, Chevrolet, Mazda, etc.).

The P066B code indicates that the Powertrain Control Module (PCM) has detected excessively high voltage in the control circuit of the glow plug for cylinder #1.

🔍 Reminder: Diesel engines use glow plugs to increase cylinder temperature and ensure proper starting, as they do not operate with sparks like gasoline engines.

⚠ Severity of the Problem

A P066B code can cause significant driving issues. It is therefore urgent to diagnose and resolve the problem quickly.

🔎 Symptoms of Code P066B

✅ Excessive black smoke from the exhaust ✅ Driving difficulties (hesitation, stumbling during acceleration) ✅ Delayed engine starting ✅ Increased fuel consumption ✅ Recorded misfire codes

🛠 Possible Causes

🔧 Faulty glow plug 🔧 Open or shorted control circuit 🔧 Loose or damaged glow plug connector 🔧 Faulty glow plug timer or control module

🏁 Troubleshooting Code P066B

1️⃣ Visual Inspection 🔍

🔹 Check all glow plug cables and connectors 🔹 Look for any damaged, melted, or disconnected wires

2️⃣ Reading Error Codes 💻

🔹 Use an OBD-II scanner to retrieve stored error codes 🔹 Note freeze frame data for analysis 🔹 Clear the codes and test the vehicle to see if the code returns

3️⃣ Testing Glow Plugs 🔥

🔹 Remove the plug and apply battery voltage 🔹 A functioning plug will become glowing red 🔹 If it does not heat up, test its resistance with a voltmeter (DVOM)

⚠ Caution: Risk of burns or fire during this test!

4️⃣ Checking Power Supply ⚡

🔹 Use a DVOM to test voltage at the glow plug connector 🔹 If no voltage is present, check the timer or control module 🔹 Test all associated fuses and relays

5️⃣ Control Circuit Check 🔄

🔹 Verify if the PCM is sending the correct voltage 🔹 If voltage is detected at the PCM output, suspect faulty wiring

🚀 Conclusion

Do not underestimate this code! A P066B can be caused by a faulty glow plug affecting the control circuit. Always verify the correct cylinder before performing in-depth diagnostics.

By following these steps, you can identify the source of the problem and resolve it effectively. 🔧🚗

P066C – Cylinder 2 Glow Plug Circuit Low

What Does It Mean?

This generic powertrain diagnostic trouble code (DTC) typically applies to many OBD-II vehicles. This may include, but is not limited to, vehicles from VW, Porsche, Ford, Toyota, GM, Chevrolet, Jeep, Chrysler, Dodge, Ram, etc.

When a P066C code is stored, it means the Powertrain Control Module (PCM) has detected a low voltage condition for the glow plug control circuit for cylinder #2. Consult a reliable vehicle information source to determine the location of cylinder #2 for your specific year/make/model/engine combination.

Diesel engines use high compression instead of spark to initiate piston movement. Since no spark is involved, the cylinder temperature must be increased to maximize compression. Glow plugs are used in each cylinder to achieve this temperature increase.

Often confused with spark plugs, a glow plug for each individual cylinder is screwed into the cylinder head. Battery voltage is applied to the glow plug element via a glow plug timer (sometimes called a glow plug controller or glow plug module) and/or the PCM. When voltage is properly applied to the glow plug, it will literally become red-hot and help raise the cylinder temperature. Once the cylinder temperature reaches the desired level, the voltage is limited by the control unit and the glow plug returns to its normal state.

If the PCM detects a voltage level for the glow plug control circuit of cylinder #2 that is lower than expected, a P066C code will be stored and a Malfunction Indicator Lamp (MIL) may be illuminated.

Example photo of a glow plug:

P066C Glow Plug
How Severe Is This DTC?

Any code related to the glow plug will likely be accompanied by driving issues. A stored P066C code should be addressed urgently.
What Are Some of the Symptoms of the Code?

Symptoms of a P066C trouble code may include:

Excessive black smoke from the exhaust
Engine running problems
Delayed engine starting
Reduced fuel efficiency
Engine misfire codes may be stored

What Are Some of the Common Causes of the Code?

Causes

of this code may include:

Faulty glow plug(s)
Open or shorted glow plug control circuit
Loose or faulty glow plug connector
Faulty glow plug timer

What Are the P066C Troubleshooting Steps?

A diagnostic scanner, a reliable vehicle information source, and a digital volt/ohmmeter (DVOM) will be necessary to reach an accurate diagnosis for a P066C code. Use the vehicle information source to look up applicable Technical Service Bulletins (TSBs). If you find a TSB that matches the vehicle’s make and model, the presented symptoms, and the stored code, it will help you make a diagnosis.

You may also need to obtain diagnostic flowcharts, wiring diagrams, connector face views, connector pinout diagrams, component locations, and component testing procedures/specifications from the vehicle information source. All this information will be needed to correctly diagnose the stored P066C code.

After performing a careful visual inspection of all glow plug and glow plug control wiring and connectors, connect the diagnostic scanner to the vehicle’s diagnostic port. Now, retrieve all stored codes and freeze frame data and record them for later (in case you need them). Next, I would test drive the vehicle to see if the P066C code resets. Drive until one of two things happens: either the PCM enters readiness mode, or the code resets. If the code resets, proceed with the diagnosis. If not, you are dealing with an intermittent condition that may need to worsen before an accurate diagnosis can be made.

Here’s a tip the service manual won’t give you. A sure way to test glow plugs is to remove them and apply battery voltage. If the glow plug becomes bright red, it is good. If the glow plug does not heat up and you take the time to test it with your DVOM, you will likely find it does not meet the manufacturer’s resistance specifications. Be careful not to burn yourself or start a fire when performing this test.

If the glow plugs appear to be working correctly, use the scanner to activate the glow plug timer and test for battery voltage (and a ground) at the glow plug connector (use the DVOM). If no voltage is present, test the power supply to the glow plug timer or glow plug controller. Test all associated fuses and relays according to the manufacturer’s recommendations. Typically, I have found it preferable to test the system fuses and fuse links with the circuit loaded. A fuse in an unloaded circuit may appear functional (when it is not) and lead you down the wrong diagnostic path.

If all fuses and relays appear functional, use the DVOM to test the output voltage at the glow plug timer or PCM (wherever they originate). If voltage is detected at the glow plug timer or PCM, suspect an open or shorted circuit. You can search for the cause of the discrepancy or simply replace the circuit.

It is sometimes thought that a P066C cannot be caused by a faulty glow plug because it is a control circuit code. Don’t be mistaken; a faulty glow plug can cause a variation in the control circuit that results in such a code.
Attempting to diagnose the wrong cylinder is more common than you think. Save yourself a major headache and make sure you are addressing the correct cylinder before starting your diagnosis.

P066D Cylinder 2 Glow Plug Circuit High

Understanding Code P066D: A Guide for Drivers

What is Code P066D?

Code P066D is a generic powertrain diagnostic code applicable to most vehicles equipped with an OBD-II system (Jeep, Chrysler, BMW, Toyota, Volkswagen, etc.). It indicates that the Powertrain Control Module (PCM) has detected an abnormally high voltage in the glow plug circuit for cylinder number 2.

Important to know: In diesel engines, glow plugs heat the cylinders to facilitate starting (no spark as in gasoline engines). If the voltage exceeds the expected threshold, the PCM records this code and illuminates the Malfunction Indicator Lamp (MIL).

Severity of Code P066D: Not to be Overlooked

This code is urgent to address:

Often causes driving problems (hard starting, black smoke, loss of power).
Can lead to misfires or damage other components.

Common Symptoms

If your vehicle displays P066D, you might notice:

🚗 Delayed starting (especially in cold weather).
⚠️ Excessive black smoke from the exhaust.
🔧 Unstable idle or loss of performance.
⛽ Increased fuel consumption.

Possible Causes

Several elements can trigger this code:

Faulty glow plug (cylinder number 2).
Electrical problem: Short circuit or open circuit in the wiring.
Damaged connector or poorly secured on the glow plug.
Controller failure (glow plug timer or PCM).

How to Diagnose the Problem?

Necessary Tools:

OBD-II scanner, digital multimeter (DVOM), vehicle-specific repair manual.

Key Steps:

Visual Inspection:

Check the cables, connectors, and fuses related to the glow plugs.

Glow Plug Testing:

Unscrew the glow plug from cylinder number 2 and apply battery voltage. If it does not glow red, replace it.
Tip: Use the DVOM to measure its resistance (compare with manufacturer specifications).

Circuit Verification:

With the scanner, activate the glow plug controller and test the voltage at the connector with the DVOM.
If there is no voltage, inspect the glow plug timer, relays, and wiring to the PCM.

Search for Intermittent Problems:

Drive the vehicle to reproduce the code. If intermittent, the fault may require more time to manifest.

Safety Tips

🔥 Caution: Glow plugs heat up extremely quickly during testing. Avoid direct contact and keep flammable materials away.
🔌 Always disconnect the battery before handling electrical components.

What to Do After Diagnosis?

If the glow plug or circuit is faulty, replace the affected parts.
Reset the code with the scanner and test the vehicle to confirm the repair.
If in doubt, consult a qualified mechanic – modern diesel systems are complex!

Summary: Code P066D signals a serious problem in the glow plug circuit. Act quickly to avoid costly damage and restore optimal performance!

P066E Cylinder 3 Glow Plug Circuit Low

DTC Code P066E: Diagnosis and Solutions

What is Code P066E?

The generic powertrain diagnostic trouble code (DTC) P066E applies to many OBD-II vehicles, including (but not limited to) those from VW, Porsche, Ford, Toyota, GM, Chevrolet, Jeep, Chrysler, Dodge, and Ram.

When a P066E code is stored, it means that the Powertrain Control Module (PCM) has detected a low voltage condition for the glow plug control circuit of cylinder #3.

Role of Glow Plugs

Diesel engines use high compression to initiate piston movement. Since no spark is involved, the cylinder temperature must be increased to maximize compression.

Each cylinder has a glow plug screwed into the cylinder head. Battery voltage is applied to these plugs via a glow plug module or timer. When voltage is correctly applied, the plug glows red and increases the cylinder temperature.

If the PCM detects a lower voltage than expected for the glow plug of cylinder #3, the P066E code is stored, and a Malfunction Indicator Lamp (MIL) may illuminate.

Severity of Code P066E

Any code related to glow plugs can lead to driving issues. It is recommended to address a P066E promptly.

Symptoms of a P066E Code

Excessive black smoke from the exhaust
Engine running problems
Delayed engine starting
Reduced fuel efficiency
Recording of engine misfire codes

Possible Causes of Code P066E

Faulty glow plug(s)
Open or shorted glow plug control circuit
Loose or faulty glow plug connector
Defective glow plug timer

Diagnostic and Repair Procedure

1. Preparation

Diagnostic scanner
Reliable source of vehicle information
Digital volt/ohmmeter (DVOM)
Electrical diagrams and connector views

2. Visual Inspection

Check glow plug wiring and connectors
Inspect related fuses and relays

3. Code Analysis

Connect the diagnostic scanner and record all codes and freeze frame data
Clear the codes and test the vehicle to see if the P066E code returns

4. Glow Plug Testing

Remove the glow plug and apply battery voltage
If it glows red, it is working. Otherwise, measure its resistance and compare to manufacturer specifications

5. Control Circuit Testing

Activate the glow plug timer with the scanner
Measure voltage at the timer input and output
If no voltage is present, test the power supply to the timer or PCM

6. Circuit Verification

If voltage is detected at the PCM or timer, look for an open or shorted circuit
Replace the circuit if necessary

Additional Tips

A P066E can be caused by a faulty glow plug, even though it is a control circuit code
Ensure diagnosing the correct cylinder to avoid wasting time

By following these steps, it is possible to identify and correct the cause of the P066E code to ensure optimal engine operation.

P066F Cylinder 3 Glow Plug Circuit High

What does it mean?

This generic powertrain diagnostic trouble code (DTC) typically applies to many OBD-II vehicles. This may include, but is not limited to, vehicles from Jeep, Chrysler, BMW, Toyota, Volkswagen, Dodge, Ram, Ford, Chevrolet, Mazda, etc.

When a P066F code is set, it means the Powertrain Control Module (PCM) has detected a high voltage situation in the glow plug control circuit for cylinder #3. Consult a reliable vehicle service resource to determine the location of said cylinder in the code description for your specific year, make, model, and engine configuration.

Often confused with spark plugs, a glow plug for each individual cylinder is screwed into the cylinder head. Battery voltage is applied to the glow plug element via a glow plug timer (sometimes called a glow plug controller or glow plug module) and/or the PCM. When voltage is properly applied to the glow plug, it will literally become glowing red and help increase the cylinder temperature. Once the cylinder temperature reaches the desired level, the voltage is limited by the control unit and the glow plug returns to its normal state.

If the PCM detects a voltage level for the glow plug control circuit of cylinder #3 that is higher than expected, a P066F code will be stored and a Malfunction Indicator Lamp (MIL) may be illuminated.

Example photo of a glow plug:

P066F Glow Plug
How severe is this DTC?

Any code related to the glow plug will likely be accompanied by driving issues. A stored P066F code should be addressed urgently.
What are some of the symptoms of the code?

Symptoms of a P066F fault code may include:

Excessive black smoke from the exhaust
Engine running problems
Delayed engine starting
Reduced fuel efficiency
Engine misfire codes may be stored

What are some of the common causes of the code?

Causes

of this code may include:

Faulty glow plug
Open or shorted glow plug control circuit
Loose or faulty glow plug connector
Faulty glow plug timer

What are the P066F troubleshooting steps?

A diagnostic scanner, a reliable source of vehicle information, and a digital volt/ohmmeter (DVOM) will be needed to reach an accurate diagnosis of a P066F code. Use the vehicle information source to look up applicable Technical Service Bulletins (TSBs). If you find a TSB that matches the vehicle’s make and model, the symptoms presented, and the stored code, it will help you make a diagnosis.

You may also need to obtain diagnostic flowcharts, wiring diagrams, connector face views, connector pinout diagrams, component locations, and component testing procedures/specifications from the vehicle information source. All of this information will be necessary to correctly diagnose the stored P066F code.

After performing a careful visual inspection of all glow plug and glow plug control wiring and connectors, connect the diagnostic scanner to the vehicle’s diagnostic port. Now, retrieve all stored codes and freeze frame data and record them for later (in case you need them). Next, I would test drive the vehicle to see if the P066F code resets. Drive until one of two things happens: either the PCM enters readiness mode, or the code resets. If the code resets, proceed with the diagnosis. If not, you are dealing with an intermittent condition that may need to worsen before an accurate diagnosis can be made.

Here’s a tip the service manual won’t give you. A sure way to test glow plugs is to remove them and apply battery voltage. If the glow plug becomes bright red, it’s good. If the glow plug does not heat up and you wish to take the time to test it with your DVOM, you will likely find that it does not meet the manufacturer’s resistance specifications. Be careful not to burn yourself or start a fire when performing this test.

If the glow plugs appear to be working correctly, use the scanner to activate the glow plug timer and test for battery voltage (and a ground) at the glow plug connector (use the DVOM). If no voltage is present, test the power supply to the glow plug timer or glow plug controller. Test all associated fuses and relays according to the manufacturer’s recommendations. As a rule, I have found it preferable to test system fuses and fuse links with the circuit loaded. The fuse for a circuit that is not loaded may appear functional (when it is not) and lead you down the wrong diagnostic path.

If all fuses and relays appear functional, use the DVOM to test the output voltage at the glow plug timer or PCM (wherever their point of origin is). If voltage is detected at the glow plug timer or PCM, suspect that you have an open or shorted circuit. You can search for the cause of the discrepancy or simply replace the circuit.

It is sometimes thought that a P066F cannot be caused by a faulty glow plug because it is a control circuit code. Don’t be mistaken; a faulty glow plug can cause a variation in the control circuit that results in such a code.
Attempting to diagnose the wrong cylinder is more common than you think. Save yourself a big headache and make sure you are addressing the correct cylinder before starting your diagnosis.

P0670 DTC – Glow Plug Control Module Circuit Malfunction

The OBD (On-Board Diagnostics) code P0670 is a generic code that applies to many modern diesel engines, including those used in vehicles from brands like Ford, Dodge, Chevrolet, GMC, and Volkswagen. To understand its meaning and implications, it is essential to master the operation of diesel engines and their specificities.

Operation of a Diesel Engine

Unlike a gasoline engine, a diesel engine does not rely on a compressed fuel-air mixture and an ignition spark to operate. Thanks to high compression, the air in the cylinder reaches a temperature above 600°C, sufficient to ignite the fuel. When a diesel engine starts, fuel is injected into the cylinder at top dead center. This fuel ignites instantly, generating gas expansion that pushes the piston downward.

Role of Glow Plugs

Starting a cold diesel engine is problematic because the compression heat can be quickly dissipated by the cold cylinder head. Glow plugs are essential to solve this problem. These pencil-like components heat up to a glow within seconds, increasing the temperature of the cylinder walls to ensure efficient combustion.

The management of glow plugs varies by vehicle:

Some use a specific control module.
Others rely on the PCM (Powertrain Control Module).

Key Elements to Check

Batteries: A full charge is essential to ensure rapid engine rotation and maintain compression heat.
Glow Plug Relay: Located near the starter relay, this component handles high currents.
Oil Temperature Sensor: This sensor tells the PCM when and how long to activate the glow plugs.
Glow Plug Fuse: Supplies power to the relay to activate the glow plugs.
Glow Plug Control Module or PCM: Determines the preheating duration based on temperature sensor information.

Symptoms of Code P0670

Glow plug indicator light turns on.
Hard starting in cold or hot weather.
Knocking noise and white smoke from the exhaust.
Misfires until the engine reaches its optimal temperature.

Potential Causes

Worn glow plugs (lifespan: 30,000 to 40,000 miles).
Faulty relay or module.
Poor injection timing, leading to premature wear.

Diagnostic Steps and Solutions

Check the Battery: Ensure it is fully charged.
Inspect the Wiring: Identify any faults or damage.
Test the Glow Plug Relay: Use a voltmeter to check the voltage at the main power terminal and detect any voltage drop (>0.5 V). Replace the relay if necessary.
Analyze the Oil Temperature Sensor: Disconnect the sensor and test the relay by turning the key.
Check the Glow Plugs: Use a test light or ohmmeter to verify their continuity. A resistance greater than 0.4 ohms indicates a problem.

Related Codes

Here are other DTC codes related to glow plugs: P0380, P0381, P0382, P0383, P0384, P0671 to P0684. These codes can provide additional clues about specific components to inspect.

By following these steps, you can diagnose and resolve issues associated with code P0670.

P0671 Cylinder 1 Glow Plug Circuit Code

Code P0671: Explanations, Symptoms, and Solutions

Meaning of Code P0671

Code P0671 is a generic OBD-II code related to a malfunction in the glow plug circuit for cylinder #1 in diesel engines. This code indicates that the Powertrain Control Module (PCM) has detected a problem in the electrical circuit or an abnormal resistance of the glow plug in cylinder 1.

Role of Glow Plugs:

🔥 Heat the cylinder head to facilitate cold starts by increasing combustion temperature.
⚡ Assist in fuel ignition in diesel engines, which rely on compression to ignite the fuel.

Severity of Code P0671

Severity Level	Potential Risks
Moderate

❄️ Difficult starting in cold weather.
📉 Loss of power or misfires.
⚠️ Engine damage if multiple glow plugs are faulty.

Recommended Action:
Although a single faulty glow plug may not prevent starting, prompt diagnosis is necessary to avoid worsening problems.

Common Symptoms

🚨 Illuminated check engine light.
❄️ Difficult or impossible starting in cold weather.
📉 Reduced power until the engine warms up.
🔧 Misfires or hesitation during acceleration.
⚡ Inactive glow plug light (does not turn off).

Possible Causes

Faulty glow plug: Resistance out of specification (0.5 to 2.0 Ω).
Electrical problem:

Damaged wires, cracked insulation, or oxidized connectors.
Short circuit or open circuit in the wiring harness.

Control module failure: Faulty relay or glow plug module.
Corrosion: Due to moisture or extreme conditions.

Diagnostic and Repair Steps

Required Tools:

📟 OBD2 scan tool to clear codes.
🔌 Digital multimeter (DVOM) to measure resistance and continuity.
🔧 Appropriate wrenches to remove the glow plug (depending on the model).

Step 1: Check for Related Codes

If code P0670 (glow plug control circuit malfunction) is present, diagnose it first.

Step 2: Visual Inspection

Locate the glow plug for cylinder #1.
Check the connector and wire:

Look for signs of corrosion, burning, or disconnection.
Ensure the wire is not chafed or damaged.

Step 3: Measure Glow Plug Resistance

Disconnect the glow plug connector.
Set the multimeter to ohmmeter (Ω) mode.
Measure the resistance between the glow plug terminal and ground:

Normal value: 0.5 to 2.0 Ω (check manufacturer specifications).
If out of range: Replace the glow plug.

Step 4: Test the Electrical Circuit

Check continuity between the relay bus bar and the glow plug:

Place one probe on the bus bar (relay side) and the other on the glow plug wire.
Resistance should be ≤ 2 Ω. If higher, replace the wire.

Inspect the fuse links on the bus bar.

Step 5: Test the Control Module

Use an advanced diagnostic tool to activate the glow plug relay.
Check the supply voltage (typically 12 V) on the circuit.
If the module does not respond, suspect an internal failure.

Step 6: Reset and Test

Clear the code with the OBD2 tool.
Perform a road test in cold weather to confirm the repair.

Good to Know

⚠️ Do not overload the glow plugs: Prolonged activation can cause them to melt.
🔄 Group replacement: On high-mileage vehicles, replace all glow plugs to avoid imbalances.
🌡️ Cold weather impact: This code is more common in winter. A faulty glow plug will make starting nearly impossible at -10°C or lower.

Conclusion

Code P0671 is often related to a worn glow plug or an electrical circuit issue. Although the engine may start with one faulty glow plug, prompt diagnosis is crucial to maintain performance and reliability. If you are not comfortable with electrical testing, consult a certified diesel mechanic.

🔧 Recommended Parts: OEM glow plugs, replacement wiring harness if necessary.

P0675 Cylinder 5 Glow Plug Circuit

What Does It Mean?

This diagnostic code (DTC) is a generic powertrain 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.

This code pertains to the device used by diesel engines to heat the cylinder head for a few seconds when attempting to start a cold engine, called a glow plug. A diesel engine relies entirely on the momentary high level of heat from compression to spontaneously ignite the fuel. The glow plug in cylinder #5 is faulty.

When a diesel engine is cold, the extremely high air temperature produced when the piston rises and compresses the air is quickly lost through heat transfer to the cold cylinder head. The solution comes in the form of a pencil-shaped preheater called a “glow plug.”

The glow plug is installed in the cylinder head very close to the point that initiates combustion, or the “hot spot.” This could be in the main chamber or the pre-chamber. When the engine management computer detects that the engine is cold using its oil and transmission sensors, it decides to assist the engine in starting by using the glow plugs.

A typical diesel engine glow plug:

Diesel engine glow plug

It grounds the glow plug timer module, which in turn grounds the glow plug relay that sends power to the glow plugs. The module powers the glow plugs. The module is usually integrated into the engine management computer, although some vehicles may have it separately.

Activating it for too long will melt the glow plugs because they produce heat through high resistance and glow red when activated. This intense heat is quickly transferred to the cylinder head, allowing the combustion heat to maintain its temperature for the fraction of a second needed to ignite the incoming fuel for starting.

The code P0675 tells you that something in the glow plug circuit is faulty and that the glow plug in cylinder #5 is not heating. The entire circuit must be checked to locate the fault.

Note: If an anomaly code P0670 is present with this DTC, diagnose P0670 before diagnosing this code.

Symptoms

With a single glow plug failure, aside from the check engine light illuminating, symptoms will be minimal because the engine will start normally with a faulty plug. In freezing conditions, you would be more likely to experience issues. The code is the primary way to identify such a problem.

    The engine management computer (PCM) will set the code P0675
    The engine will be hard to start or may not start at all in cold weather or when it has been sitting long enough for the block to cool.
    Lack of power until the engine warms up sufficiently.
    The engine may misfire due to the cylinder head temperature being colder than normal
    The engine may hesitate during acceleration
    No preheating period, i.e., the glow plug light does not turn off.

Potential Causes

Causes

of this DTC may include:

    Faulty glow plug in cylinder #5
    Open or shorted glow plug circuit
    Damaged wiring connector
    Faulty glow plug control module

Diagnostic Steps

and Possible Solutions

You will need a digital volt-ohmmeter (DVOM) for a complete test. Continue testing until the problem is confirmed. You will also need a basic OBD code scanner to reset the computer and clear the code.

Check the glow plug in cylinder #5 by disconnecting the wire connector on the plug. Set the DVOM to ohms and place the red lead on the glow plug terminal and the black lead on a good ground. The range is 0.5 to 5.0 ohms (check your application’s specification by referring to a factory service manual). If it is out of range, replace the glow plug.

Check the resistance of the glow plug wire to the glow plug relay bus bar on the valve cover. Note that the relay (looks like a starter relay) has a large-gauge wire leading to a bus bar where all the glow plug wires are attached. Test the glow plug wire for number one by placing the red lead on the number one wire at the bus bar and the black lead on the glow plug side. Again, 0.5 to 5.0 ohms, with 5 ohms being the highest resistance. If it is higher, replace the glow plug wire from the bus bar. Also note that these wires from the bus bar to the plugs are fuse links. Reconnect the wires.

Check the same wires for looseness, cracks, or missing insulation. Connect the code scanner to the OBD port under the dashboard and turn the key to the “on” position with the engine off. Clear the codes.