P0787 Fault Code in Toyota: Causes, Symptoms, and Fixes

This code means the Transmission Control Module (TCM) has detected an electrical signal lower than expected in the circuit for Shift/Timing Solenoid A. Essentially, the solenoid is not receiving enough voltage/current, or it’s stuck in a low state, which disrupts proper gear shifting.

Possible Causes:

1. Low ATF level or dirty/burnt fluid
2. Faulty Shift Solenoid A (stuck, shorted, or internally damaged)
3. Open or short-to-ground in solenoid wiring
4. Corroded or loose electrical connectors
5. Bad transmission control relay or fuse
6. TCM failure (rare)

Symptoms:

1. Harsh or delayed gear changes
2. Transmission stuck in one gear (limp mode)
3. Slipping or skipping gears
4. Poor acceleration, reduced drivability
5. Check Engine Light (CEL) or Transmission Warning Light ON

Common Fixes:

1. Check ATF fluid – refill/replace if low, dirty, or burnt.
2. Inspect wiring/connectors to solenoid A – repair/replace as needed.
3. Test Shift Solenoid A – measure resistance, check actuation, replace if faulty.
4. Check transmission control relay/fuses.
5. Inspect valve body – clean or repair if hydraulic passages are blocked.
6. Replace/reprogram TCM if electrical/mechanical issues are ruled out.