P0787 indicates that the PCM/TCM has detected a low voltage condition in the circuit controlling the shift timing solenoid. This solenoid regulates hydraulic pressure for gear changes, and a low signal can prevent proper shifting.
The solenoid itself may have an internal short or open coil, causing the control circuit to read low voltage.
Damaged, corroded, or loose wiring harness or connector pins for the shift timing solenoid can create a short to ground or open circuit, lowering the sensed voltage.
Insufficient fluid level or degraded fluid reduces hydraulic pressure, which can cause the PCM/TCM to interpret the solenoid command as low voltage.
Debris or wear inside the transmission valve body can impede solenoid movement, leading to abnormal pressure feedback and a low‑voltage fault code.
Although uncommon, a malfunction in the TCM/PCM driver circuit for the shift timing solenoid can produce a false low‑voltage reading; this is considered only after other causes are ruled out.
No. The PCM/TCM is rarely at fault for this code. Most P0787 cases are due to a failed shift timing solenoid, wiring/connector problems, or fluid-related issues. PCM replacement should only be considered after those components have been tested and found good.
Yes. Low fluid reduces hydraulic pressure, which can lead the PCM/TCM to interpret the shift timing solenoid circuit as low voltage, triggering the code.
Inspect the wiring harness and connectors for intermittent faults, check for valve‑body blockage or sticking, and verify fluid pressure. If those are all normal, further diagnosis of the TCM/PCM driver circuit may be required.