P0766 Code: Shift Solenoid “D” Performance/Stuck Off

Shift solenoids control the flow of hydraulic fluid through an automatic transmission. Taking their cue from the transmission control module (TCM), they adjust the transmission’s hydraulic pressure to engage the clutch pack with the right gear ratio. They enable the TCM to shift the transmission into the right gear ratio to get the most engine performance out of as little RPM as possible. 

The TCM will log a P0766 code if it determines that the shift solenoid in the D circuit is not performing as designed and desired.

What Does the P0766 Code Mean?

The diagnostic trouble code (DTC) P0766 indicates “Shift Solenoid “D” Performance/Stuck Off.” It applies to the solenoid in the D circuit.

In most automatic transmissions, each gear has one shift solenoid associated with it. Each solenoid also has a circuit through which the TCM can keep track of the device.

The shift solenoids control how much transmission fluid flows between different hydraulic circuits. Each solenoid is in charge of a valve in the valve body, which leads to a clutch pack.

When it opens its associated valve, hydraulic fluid enters the clutches and bands that change the transmission gear. The hydraulic pressure applied by the fluid shifts the transmission into a different gear.

The TCM regularly checks its shift solenoid circuits for any problems, such as a stuck scenario. If the computer doesn’t get the right response from the D circuit, it will set the generic transmission code P0766.

Note: The definition of code P0766 can differ according to the vehicle manufacturer. Check the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of a Code P0766?

• Low transmission fluid level
• Faulty transmission shift solenoid
• Wiring problems, including grounded, shorted, and corroded wires
• Bad TCM

What are the Common Symptoms of a Code P0766?

• Illuminated check engine light
• Higher fuel consumption
• Transmission doesn’t shift correctly

How to Diagnose the P0766 Code

The P0766 code is a generic transmission code that various makes and models can log. However, this does not mean that there is a one-size-fits-all solution to identify its underlying problem.

If you’re not confident about conducting diagnostic tests on your vehicle, it’s a good idea to bring it to a professional mechanic. Otherwise, you can go ahead and diagnose the issue yourself. To help you get started, here is a video that can give you an idea of what the process might involve:

How to Fix the P0766 Code

Attempting to fix a P0766 code can become a challenging task if you don’t have the right tools and know-how to test the TCM’s shift solenoids. In most cases, it’s best to leave the job to professionals.

For DIYers who would rather test and replace their vehicle’s TCM pressure control solenoid on their own, it will be helpful to rely on the appropriate repair manual or refer to an online repair database.