Error Code P078D is defined as Shift Timing Solenoid B High. This is a generic trouble code, meaning it applies to all vehicles with the OBD-II system, especially those made since 1996 up to present and equipped with automatic transmission. Specifications on the definition and steps for repairs may vary depending on the model and make of the vehicle.

This includes vehicles from, but not limited to, BMW, Chevrolet, Dodge, Ford, GMC, Saab, Subaru, Toyota, and Volkswagen. Specifications on the definition, troubleshooting, and repairs, of course, vary between make, model and powertrain configuration.

The automatic transmission shift seamlessly depending on the vehicle’s driving needs. It does its job with the help of multiple components, including the shift timing solenoid. The Transmission Control Module (TCM) monitors and controls the functions of this solenoid.

The main job of the solenoid is to control the internal hydraulic (Automatic Transmission Fluid or ATF) flow and assist the shifting of the transmission. These solenoids are electro-mechanical “pistons”. Most of the time, there are multiple shift timing solenoids inside the system, which means you will have to refer to the letter included in the code to pinpoint which of the specific solenoid you should work with, depending on the make and model of the vehicle.

The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) activates the Check Engine light and stores the error code. Other related error codes include:

  • Error Code P078A
  • Error Code P078B
  • Error Code P078C
  • Error Code P078E

The circuit includes the TCM, and depending on the model of the vehicle, it could be placed in a volatile position.

A mechanical or electrical problem could cause this code. Error Code P078D is set when the PCM and/or TCM detects a specific high electrical value within the shift timing solenoid B circuit. This means there’s a problem with the shift/timing solenoid, as it is registering higher than the allowed preset parameters.

The automatic transmission is intricately designed and can be complicated for DIYers. Thus, it is best to leave this problem to the professionals.

Common Symptoms

  • Erratic, hard, or harsh shifting
  • Inefficient shit timing
  • Stuck gear
  • Transmission slippage
  • Poor acceleration and drivability
  • Poor overall performance

Possible Causes

  • Damaged or faulty shift timing solenoid
  • Obstruction or blocked transmission fluid passages
  • Dirty ATF (Automatic Transmission Fluid)
  • Low of dirty transmission fluid
  • Open or shorted solenoid valve harness
  • Defective PCM or TCM
  • Faulty electrical connection at shift/timing solenoid valve circuit

How to Check

Before starting the troubleshooting, you should have the Technical Service Bulletin (TSB) for the specific make, model, and transmission of your vehicle. This step can help you save time and money for the diagnosis and repair process.

Check the ATF. Dirty, smelly, abnormally colored fluid must be replaced. Also, note that too much transmission fluid is just as bad as too little.

Note that old and dirty can cause blockage, which can be the cause of the error code. Thus, it is important to follow the recommended transmission fluid maintenance of the vehicle.

If the fluid checks out good, then check the shift solenoid. Depending on the make and model of the vehicle, this can be accessed externally. Check the solenoid for signs of damage, including the connectors and harnesses. Replace or repair as necessary. Also, inspect for any leaks.

For internal solenoid, it’s highly recommended that you leave the problem to the professional. Try to exhaust other solutions, as diagnosis and repairs for the internal transmission must be done by a certified technician.

Also, both the TCM and the harness are usually subjected to harsh road conditions, so make sure they’re intact and in good shape.

Next, test the shift timing solenoid. You can shift the solenoid in multiple ways. First, check for resistance using a multimeter. Measure the resistance between the pins and the solenoids and verify the electrical integrity. Then, apply voltage to the solenoid manually to check its mechanical operation. This should be either specified by the manufacturer or in the service manual/technical literature of your vehicle.

Then, test the circuit from the TCM. You can also verify the functionality of the sensor along with its circuit by conducting the same tests as mentioned above for the pins at the PCM and TCM. This is a good way to verify not just the integrity of the solenoid, but the electrical integrity of the circuit as well.

How to Fix

  • Replacement of shift solenoids
  • Flushing and replacement of transmission fluid
  • Addition of transmission fluid
  • Replacement of defective internal components in the transmission
  • Unblocking of blocked hydraulics and patching of leaks
  • Replacement of PCM (rare)

Though this code is moderately severe, it is best to address it as soon as possible. Leaving it for too long could lead to a more serious problem to the transmission.