P0788 means the PCM/TCM has sensed a voltage higher than the expected range in the circuit for the shift timing solenoid. This solenoid regulates hydraulic pressure to control the timing of gear shifts. A high‑voltage condition usually indicates an electrical fault in the solenoid, its wiring, connectors, or, rarely, the control module itself.
The solenoid itself may have an internal short to power or excessive resistance, causing the PCM to read a voltage above the normal threshold.
Damaged insulation or chafed wires in the solenoid circuit can allow battery voltage to feed the signal line, producing a high‑voltage reading.
Poor contact or corrosion can create erratic voltage readings that the PCM interprets as a high signal.
Insufficient fluid pressure or contaminated fluid can cause the solenoid to work harder, sometimes leading to abnormal voltage feedback.
While possible, a defective control module is an uncommon cause; it should be considered only after the solenoid, wiring, connectors, and fluid have been verified as good.
No. The PCM/ECM is rarely at fault for this code. Most cases are due to the shift timing solenoid, its wiring, connectors, or transmission fluid condition. PCM replacement should only be considered after those items have been ruled out.
Driving is possible but may cause harsh shifting, reduced fuel economy, or the transmission to enter limp‑in mode. Continued operation could lead to further transmission wear, so it is advisable to diagnose and repair the issue promptly.
Clearing the code only turns off the Check Engine light; if the underlying electrical or hydraulic fault remains, the code will return once the PCM detects the abnormal voltage again.