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 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 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?
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 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!
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 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 manual/technical documentation.
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.