P0782 2-3 Shift Malfunction

What does it mean?

In electronically controlled automatic transmission vehicles, the 2-3 shift solenoid is responsible for actuating hydraulic circuits to engage clutches or bands that shift gears inside the automatic transmission. Shift solenoids are controlled either by a Transmission Control Module (TCM) or by the Engine Control Unit/Powertrain Control Module (ECU/PCM). The control circuit can be power-controlled or ground-controlled depending on the application.

Some vehicles automatically test the circuit even when the gear is not in use; the 2-3 shift solenoid can be an on/off solenoid or a duty cycle controlled solenoid. The PCM or TCM looks for an inductive kick to determine if the solenoid is functioning correctly even when it is not in use. This brief activation is too quick for the solenoid to actually cause a shift in the transmission. When the solenoid is turned on and off, the magnetic field collapses, sending a slightly higher voltage to the PCM, which is used to determine if the solenoid is working properly. Note: This DTC is almost identical to P0781, P0783, P0784, and P0829.

Symptoms

Symptoms of a DTC P0782 may include:

Check Engine Light illuminated (Malfunction Indicator Lamp/MIL)
The vehicle will not shift from 2nd to 3rd gear

This condition may turn on the Check Engine Light and possibly disable transmission controls entirely, resulting in a failsafe mode that will only allow one default gear depending on the vehicle’s condition with all solenoids deactivated.

Causes

Potential causes of a P0782 code include:

2-3 shift solenoid failure
Electrical short to power – pinched wires
Electrical short to ground – pinched wires
Open power or ground circuit wiring – disconnected or loose connections
Internal TCM or PCM failure

Possible Solutions

Disconnect the PCM or TCM wiring harness, depending on the application, and using an electrical diagram as a reference, check for excessive resistance or an open in the control circuit. Using a Digital Volt-Ohm Meter (DVOM) set to the Ohms scale, check the resistance on the 2-3 solenoid’s power and ground sources to determine if there is an open circuit or excessive resistance in the 2-3 solenoid. Visually inspect for pinched, disconnected, or loose wires on the harness connectors.

Short to Power: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the power source and the solenoid circuit wiring with the key in the On/Engine Off (KOEO) position as well as with the key off.

Short to Ground: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the 2-3 solenoid power source and a known good ground.

TCM or PCM Failure: Using an advanced scan tool capable of controlling solenoids, manually activate the 2-3 shift solenoid. It may be necessary to manually activate the transmission control relay on some vehicles if power is not present at the transmission wiring harness connector. If no scan tool is available, it will be necessary to monitor the 2-3 solenoid circuit to determine if the TCM or PCM is sending a power or ground signal to the 2-3 solenoid during vehicle operation.

P0783 3-4 Gear Shift Malfunction

What does it mean?

In vehicles with electronically controlled automatic transmissions, the 3-4 shift solenoid is responsible for actuating the hydraulic circuits to engage the clutches or bands that change gears inside the automatic transmission. Shift solenoids are controlled either by a Transmission Control Module (TCM) or by the Engine Control Unit/Powertrain Control Module (ECU/PCM). The control circuit can be power-controlled or ground-controlled depending on the application.

Some vehicles automatically test the circuit even when the gear is not in use; the 3-4 shift solenoid can be an on/off solenoid or a duty cycle controlled one. The PCM or TCM looks for an inductive kick to determine if the solenoid is functioning correctly even when it is not in use. This brief activation is too quick for the solenoid to actually cause a shift in the transmission. When the solenoid is activated and deactivated, the magnetic field collapses, sending a slightly higher voltage to the PCM, which is used to determine if the solenoid is working properly. Note: This DTC is almost identical to P0781, P0782, P0784, and P0829.

Symptoms

Symptoms of a DTC P0783 may include:

Check Engine Light illuminated (Malfunction Indicator Lamp/MIL)
The vehicle will not shift from 3rd to 4th gear

This condition may turn on the check engine light and possibly disable the transmission controls entirely, resulting in a fail-safe mode that will only allow a single default gear depending on the vehicle’s condition with all solenoids deactivated.

Causes

Potential causes of a P0783 code include:

3-4 shift solenoid failure
Electrical short to power – pinched wires
Electrical short to ground – pinched wires
Open power or ground circuit wiring – disconnected or loose connections
Internal TCM or PCM failure

Possible Solutions

Disconnect the PCM or TCM wiring harness, depending on the application, and using an electrical diagram as a reference, check for excessive resistance or an open in the control circuit. Using a Digital Volt-Ohm Meter (DVOM) set to the Ohms scale, check the resistance on the 3-4 solenoid power and ground sources to determine if there is an open circuit or excessive resistance in the 3-4 solenoid. Visually inspect for pinched, disconnected, or loose wires on the harness connectors.

Short to power: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the power source and the solenoid circuit wiring in Key On Engine Off (KOEO) position as well as in Key Off position.

Short to ground: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the 3-4 solenoid power source and a known good ground.

TCM or PCM failure: Using an advanced scan tool capable of controlling solenoids, manually activate the 3-4 shift solenoid. It may be necessary to manually activate the transmission control relay on some vehicles if power is not present at the transmission wiring harness connector. If no scan tool is available, it will be necessary to monitor the 3-4 solenoid circuit to determine if the TCM or PCM is sending a power or ground signal to the 3-4 solenoid during vehicle operation.

P0784 4-5 Gear Shift Malfunction

What does it mean?

In electronically controlled automatic transmission vehicles, the 4-5 shift solenoid is responsible for actuating hydraulic circuits to engage the clutches or bands that shift gears inside the automatic transmission. Shift solenoids are controlled either by a Transmission Control Module (TCM) or by the Engine Control Unit/Powertrain Control Module (ECU/PCM). The control circuit can be power-controlled or ground-controlled depending on the application.

Some vehicles automatically test the circuit even when the gear is not in use; the 4-5 shift solenoid may be an on/off solenoid or a duty cycle controlled solenoid. The PCM or TCM looks for an inductive kick to determine if the solenoid is functioning correctly even when it is not in use. This brief activation is too quick for the solenoid to actually cause a shift in the transmission. When the solenoid is turned on and off, the magnetic field collapses, sending a slightly higher voltage to the PCM, which is used to determine if the solenoid is operating properly. Note: This DTC is nearly identical to P0781, P0782, P0783, and P0829.

Symptoms

Symptoms of a DTC P0784 may include:

Check Engine Light is on (Malfunction Indicator Lamp/MIL)
The vehicle will not shift from 4th to 5th gear

This condition may illuminate the Check Engine Light and possibly disable transmission controls entirely, resulting in a fail-safe mode that will only allow a single default gear depending on the vehicle’s condition with all solenoids deactivated.

Causes

Potential causes of a P0784 code include:

    Failure of the 4-5 shift solenoid
    Electrical short to power – pinched wires
    Electrical short to ground – pinched wires
    Open circuit in power or ground wiring – disconnected or loose connections
    Internal TCM or PCM failure

Possible Solutions

Disconnect the PCM or TCM wiring harness, depending on the application, and using an electrical diagram as a reference, check for excessive resistance or an open in the control circuit. Using a Digital Volt-Ohm Meter (DVOM) set to the Ohms scale, check the resistance on the 4-5 solenoid power and ground sources to determine if there is an open circuit or excessive resistance in the 4-5 solenoid. Visually inspect for pinched, disconnected, or loose wires on the harness connectors.

Short to Power: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the power source and the solenoid circuit wiring in Key On Engine Off (KOEO) position as well as in Key Off position.

Short to Ground: Disconnect the positive battery cable and the wiring harness connector at the transmission and check for continuity using the DVOM between the 4-5 solenoid power source and a known good ground.

TCM or PCM Failure: Using an advanced scan tool capable of controlling solenoids, manually activate the 4-5 shift solenoid. It may be necessary to manually activate the transmission control relay on some vehicles if power is not present at the transmission wiring harness connector. If no scan tool is available, it will be necessary to monitor the 4-5 solenoid circuit to determine if the TCM or PCM is sending a power or ground signal to the 4-5 solenoid during vehicle operation.

P0785 Shift Timing Solenoid A Malfunction

What does it mean?

This is a generic diagnostic trouble code (DTC) and generally applies to OBD-II equipped vehicles with an automatic transmission. Vehicle makes may include, but are not limited to, Chevrolet/Chevy, Volvo, GMC, Saab, Subaru, VW, BMW, Toyota, Ford, Dodge, etc.

The main purpose of the solenoids is to control the hydraulic flow (ATF: Automatic Transmission Fluid) internally to help move the transmission. These solenoids are essentially electromechanical “pistons.” Most of the time, there are multiple shift timing solenoids in the system, so you can refer to the letter included in the code to identify the specific solenoid you are working with based on your particular make and model.

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P0785 and related codes (P0786, P0787, P0788, and P0789) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and whether others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the professionals, especially when it comes to internal transmission work.

The P0785 code “Shift Timing Solenoid A Malfunction” is set when there is a general malfunction present in the solenoid or its circuit.

Note: If you have a GM vehicle (GMC/Chevrolet/etc.) with the 4L60-E transmission, the code description is actually for the 3-2 shift solenoid.
How severe is this DTC?

I would say this code is moderately severe. If this issue is not fixed for too long, you risk seriously damaging the transmission, not to mention your wallet.
What are some of the symptoms of the code?

Symptoms of a P0785 trouble code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What are some common causes of the code?

Causes

of this P0785 transmission code may include:

Damaged or faulty shift timing solenoid
Dirty automatic transmission fluid (ATF)
Low ATF
Obstruction in ATF passages
Broken or damaged wiring harness
TCM issue
ECM issue
Pin/connector problem (corrosion, melting, broken locking tab, etc.)

What are the troubleshooting steps for P0785?

Before starting the troubleshooting process for any malfunction, you should look up Technical Service Bulletins (TSBs) for the specific vehicle by year, model, and transmission. This step can save you time and money during diagnosis and repair!
Basic Step #1

The first thing I would do here is check the ATF (Automatic Transmission Fluid). If the fluid is dirty, smells burnt, is overfilled, and/or has an abnormal color, it might be time to change your fluid. If you have owned your vehicle for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) and don’t remember changing your fluid or have records of it, not only could this be your problem, but it could have been avoided simply by following your maintenance schedule. Just something to think about! Old fluid is dirty fluid, dirty fluid can cause blockages, and blockages can cause this code, so change your fluid!

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

Is the fluid good, the code still active, and symptoms present? Depending on your specific model, the shift timing solenoid may be accessible from the outside. If so, inspect the solenoid and its harness/connectors for any signs of damage. Repair/replace any issues if necessary. This is also a good time to check for any leaks!

TECH TIP: If your shift timing solenoid is internal, I do not recommend trying to access it; skip it for now. Exhaust other avenues; diagnosing and repairing internal transmissions should be left to a certified technician.

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

Test the shift timing solenoid. You may be able to test the solenoid itself in several ways. One test might include a resistance test where you use a multimeter to measure the resistance between the pins in the solenoid to check its electrical integrity to some extent. Another might involve manually applying voltage to the solenoid to check its mechanical operation. Either should be specified in your manufacturer’s service/technical manual.
Basic Step #4

Test the circuit from the TCM. You may be able to check the sensor’s functionality and its circuit by performing the same tests as above but from the TCM and/or ECM pins. This is a good trick to check not only the solenoid’s integrity but also the electrical integrity of the circuit.

P0786 Range Shift Synchronization Solenoid A / Performance

What does it mean?

The main purpose of the solenoids is to control the internal hydraulic flow (ATF: Automatic Transmission Fluid) to help move the transmission. These solenoids are essentially electromechanical “pistons.” Most of the time, there are multiple shift timing solenoids in the system, so you can refer to the letter included in the code to identify the specific solenoid you are working with based on your particular make and model.

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P0786 and related codes (P0785, P0787, P0788, and P0789) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and if others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The P0786 shift range A timing solenoid code is set when the ECM (Engine Control Module) and/or TCM (Transmission Control Module) detects that the solenoid is operating out of range or abnormally.
How severe is this DTC?

Symptoms of a P0786 trouble code may include:

Stuck in gear
Erratic shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What are some of the common causes of the code?

Causes

of this P0786 transmission code may include:

Damaged or faulty shift timing solenoid
Dirty automatic transmission fluid (ATF)
Low ATF
Blockage in ATF passages
Broken or damaged wiring harness
TCM problem
ECM problem
Pin/connector issue (corrosion, melting, broken locking tab, etc.)

What are the troubleshooting steps for P0786?

Before starting the troubleshooting process for any malfunction, you should look up the technical service bulletins (TSBs) for the specific vehicle by year, model, and transmission. This step can save you time and money during diagnosis and repair!
Basic Step #1

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

Is the fluid good, the code still active, and symptoms present? Depending on your specific model, the shift solenoid might be accessible from the outside. If so, inspect the solenoid and its harness/connectors for any signs of damage. Repair/replace any issues as needed. This is also a good time to check for any leaks!

TECH TIP: If your shift solenoid is internal, I do not recommend trying to access it; skip it for now. Exhaust other avenues; diagnosing and repairing internal transmissions should be left to a certified technician.

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

Test the circuit from the TCM. You might be able to check the sensor’s functionality and its circuit by performing the same tests as above but from the TCM and/or ECM pins. This is a good trick to check not only the solenoid’s integrity but also the electrical integrity of the circuit.

P0787 Shift Timing Solenoid A Low

What Does It Mean?

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P0787 and related codes (P0785, P0786, P0788, and P0789) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and if others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The P0787 Shift Timing Solenoid A Low code is defined when the ECM (Engine Control Module) and/or TCM (Transmission Control Module) detects a specific low electrical value in the Shift Timing Solenoid A circuit.
How Severe Is This DTC?

I would say this code is moderately severe. If this issue is not fixed for too long, you risk seriously damaging the transmission, not to mention your wallet.
What Are Some of the Symptoms of the Code?

Symptoms of a P0787 error code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What Are Some of the Common Causes of the Code?

Causes

of this P0787 transmission code may include:

Damaged or faulty shift timing solenoid
Dirty automatic transmission fluid (ATF)
Low ATF
Blockage in ATF passages
Broken or damaged wiring harness
TCM issue
ECM issue
Pin/connector problem (corrosion, melting, broken locking tab, etc.)

What Are the Troubleshooting Steps for P0787?

The first thing I would do here is check the ATF (Automatic Transmission Fluid). If the fluid is dirty, smells burnt, is overfilled, and/or has an abnormal color, it may be time to change your fluid. If you have owned your vehicle for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) and don’t remember changing your fluid or have records of it, not only could this be your problem, but it can be avoided simply by following your maintenance schedule. Just something to think about! Old fluid is dirty fluid, dirty fluid can cause blockages, and blockages can cause this code, so change your fluid!

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

Is the fluid good, the code still active, and symptoms present? Depending on your specific model, the shift solenoid may be accessible from the outside. If so, inspect the solenoid and its harness/connectors for any signs of damage. Repair/replace any issues as needed. This is also a good time to check for any leaks!

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

P0788 High Shift Timing Solenoid A

What Does It Mean?

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P0788 and related codes (P0785, P0786, P0787, and P0789) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and whether others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The P0788 Shift Timing Solenoid A High code is set when the ECM (Engine Control Module) and/or TCM (Transmission Control Module) detects a specific high electrical value in the Shift Timing Solenoid A circuit.
How Severe Is This DTC?

Symptoms of a P0788 trouble code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What Are Some of the Common Causes of the Code?

Causes

of this P0788 transmission code may include:

Damaged or faulty shift timing solenoid
Dirty automatic transmission fluid (ATF)
Low ATF
Obstruction in ATF passages
Broken or damaged wiring harness
TCM problem
ECM problem
Pin/connector issue (corrosion, melting, broken locking tab, etc.)

What Are the Troubleshooting Steps for P0788?

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

Is the fluid good, the code still active, and symptoms present? Depending on your specific model, the shift timing solenoid may be accessible from the outside. If so, inspect the solenoid and its harness/connectors for any signs of damage. Repair/replace any issues as needed. This is also a good time to check for any leaks!

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

Test the circuit from the TCM. You may be able to

P0789 Shift Timing Solenoid A Intermittent

What does it mean?

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P0789 and related codes (P0785, P0786, P0787, and P0788) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and whether others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The P0789 Shift Timing Solenoid A Intermittent code is set when the ECM (Engine Control Module) and/or TCM (Transmission Control Module) detects a fault that is not consistent in the Shift Solenoid A circuit.
How severe is this DTC?

Symptoms of a P0789 trouble code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What are some common causes of the code?

Causes

of this P0789 transmission code may include:

What are the troubleshooting steps for P0789?

Before starting the troubleshooting process for any malfunction, you should search for Technical Service Bulletins (TSBs) for the specific vehicle by year, model, and transmission. This step can save you time and money during diagnosis and repair!
Basic Step #1

NOTE: Follow the manufacturer-recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so ensure everything is intact and as it should be.
Basic Step #3

P078A Shift Timing Solenoid B Malfunction

What does it mean?

The shift timing solenoid is one of many components that allow an automatic transmission to shift smoothly according to your driving needs. The TCM (Transmission Control Module) monitors and controls the functionality of the solenoids.
The main purpose of the solenoids is to control the internal hydraulic flow (ATF: Automatic Transmission Fluid) to help move the transmission. These solenoids are essentially electromechanical “pistons.” Most of the time, there are multiple shift timing solenoids in the system, so you can refer to the letter included in the code to identify the specific solenoid you are working with based on your particular make and model.

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P078A and related codes (P078B, P078C, P078D, and P078E) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and whether others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The code P078A Shift Timing Solenoid B Dysfunction is set when there is a general malfunction present in the solenoid or its circuit.
How severe is this DTC?

I would say this code is moderately severe. If this issue is not fixed for too long, you risk seriously damaging the transmission, not to mention your wallet.
What are some of the symptoms of the code?

Symptoms of a P078A trouble code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What are some of the common causes of the code?

Causes

of this P078A transmission code may include:

What are the troubleshooting steps for P078A?

The first thing I would do here is check the ATF (Automatic Transmission Fluid). If the fluid is dirty, smells burnt, is overfilled, and/or has an abnormal color, it may be time to change your fluid. If you have owned your vehicle for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) and don’t remember changing your fluid or have records of it, not only could this be your problem, but it could have been avoided simply by following your maintenance schedule. Just something to think about! Old fluid is dirty fluid, dirty fluid can cause blockages, and blockages can cause this code, so change your fluid!

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

Is the fluid good, the code still active, and symptoms present? Depending on your specific model, the shift solenoid may be accessible from the outside. If so, inspect the solenoid and its harness/connectors for any signs of damage. Repair/replace any issues as necessary. This is also a good time to check for any leaks!

NOTE: The TCM and harness involved in this circuit may be subjected to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

Test the circuit from the TCM. You may be able to check the functionality of the sensor and its circuit by performing the same tests as above but from the TCM and/or ECM pins. This is a good trick to check not only the solenoid’s integrity but also the electrical integrity of the circuit.

P078B Range Shift Synchronization Solenoid B / Performance

What does it mean?

This is a generic diagnostic trouble code (DTC) and generally applies to OBD-II equipped vehicles with automatic transmissions. Vehicle makes may include, but are not limited to, Chevrolet/Chevy, Volvo, GMC, Saab, Subaru, VW, BMW, Toyota, Ford, Dodge, etc.

The ECM (Engine Control Module) illuminates the MIL (Malfunction Indicator Lamp) with P078B and related codes (P078A, P078C, P078D, and P078E) when it detects one or more issues in the shift timing solenoid or its circuit. The circuit includes the TCM, which, depending on the model, might be located in a volatile area. There are many possible causes here, so it could be mechanical and/or electrical depending on the code and whether others are active. Automatic transmissions are complexly designed systems, and in my opinion, the layperson or average DIYer should leave this to the pros, especially when it comes to internal transmission work.

The P078B Shift Timing Solenoid B Range/Performance code is set when the ECM (Engine Control Module) and/or TCM (Transmission Control Module) detects that the solenoid is operating out of range or abnormally.
How severe is this DTC?

Symptoms of a P078B trouble code may include:

Stuck in gear
Irregular shifting
Transmission slipping
Hard or harsh shifting
Inefficient timing
Poor driving
Poor acceleration
Poor overall performance

What are some common causes of the code?

Causes

of this P078B transmission code may include:

Before starting the troubleshooting process for any malfunction, you should look up technical service bulletins (TSBs) for the specific vehicle by year, model, and transmission. This step can save you time and money during diagnosis and repair!
Basic Step #1

The first thing I would do here is check the ATF (Automatic Transmission Fluid). If the fluid is dirty, smells burnt, is overfilled, and/or has an abnormal color, it may be time to change your fluid. If you’ve owned your vehicle for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) and don’t remember changing your fluid or have records of it, not only could this be your problem, but it could have been avoided simply by following your maintenance schedule. Just something to think about! Old fluid is dirty fluid, dirty fluid can cause blockages, and blockages can cause this code, so change your fluid!

NOTE: Follow the manufacturer’s recommended steps when checking the ATF. This simple step can vary significantly between makes and models.
Basic Step #2

NOTE: The TCM and harness involved in this circuit may be exposed to harsh road conditions, so make sure everything is intact and as it should be.
Basic Step #3

Test the shift timing solenoid. You may be able to test the solenoid itself in several ways. One test might involve a resistance test where you use a multimeter to measure the resistance between the solenoid pins to check its electrical integrity to some extent. Another might involve manually applying voltage to the solenoid to check its mechanical operation. Either should be specified in your manufacturer’s service manual/technical documentation.
Basic Step #4