P0766 code definition
The P0766 code indicates that there has been an abnormal voltage reading detected by the PCM in the shift solenoid D.
What the P0766 code means
Shift solenoids are used to managed the hydraulic fluid transfer between circuits, and to assist in the process that allows a vehicle to decelerate and accelerate. These solenoids must be able to actuate varying gear ratios to achieve vehicle speed and engine performance. If a solenoid (in this case, shift solenoid D) is not performing correctly, or is stuck in the off position, the engine will not perform at optimal level. In this case, the PCM will store the P0766 code.
What causes the P0766 code?
The most likely causes include:
- Faulty shift solenoid
- Blocked transmission fluid passages
- Internal failures in the transmission
- Low, dirty, or contaminated transmission fluid
- Open or short shift solenoid D valve harness
- Poor electrical connection within the solenoid D circuit
- A faulty PCM or TCM, in very rare cases
What are the symptoms of the P0766 code?
In many cases, there are no symptoms at all beyond the illumination of the Check Engine lamp. However, some vehicle owners may observe:
- Harsh shifting.
- Transmission slippage.
- Overheated transmission.
- Decreased fuel economy.
- Failure to shift into or out of a particular gear.
How does a mechanic diagnose the P0766 code?
After verifying the code with an OBD-II scanner, the mechanic should begin by inspecting the transmission fluid. If the fluid is too low, dirty, or smells burnt, it needs to be refilled or replaced with clean fluid. In the event that the transmission fluid is heavily contaminated, the entire system may need to be flushed and refilled.
The next step is to inspect all electrical components, and replace or repair any that are damaged, loose, corroded, or otherwise inoperable. After this step, a mechanic should check the hydraulic pressure, the shift solenoid function, and all related circuits until the improperly functioning component is found.
After every attempted repair is complete, the P0766 code should be cleared from the system, and a retest should be performed. If the code does not reappear, the mechanic has successfully diagnosed the issue, and will know precisely which component was causing the issue.
Common mistakes when diagnosing the P0766 code
The most common mistake made when diagnosing the P0766 code is jumping right to the shift solenoid itself, and replacing that component, rather than first checking the circuit components for electrical issues.
How serious is the P0766 code?
If the P0766 code is being detected, it indicates an issue that directly affects the operation of the vehicle. Even when symptoms are not present, this code means that the vehicle may not be operation for much longer, or that more expensive repairs will be needed down the road. If the P0766 code is detected, it should be dealt with immediately.
What repairs can fix the P0766 code?
There are many ways that a technician can address a P0766 code detection. They include:
- Replacing faulty shift solenoid
- Clearing blocked transmission fluid passages
- Repairing internal failures in the transmission
- Refilling low, dirty, or contaminated transmission fluid
- Replacing open or short shift solenoid D valve harness
- Repairing poor electrical connection within the solenoid D circuit
- Replacing faulty PCM or TCM, in very rare cases
Need help with a P0766 code?
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