P0738 indicates that the transmission control module (TCM) has detected a low voltage condition on its engine speed output circuit. This circuit sends engine RPM data to other modules, and a low signal can cause erratic shifting, limp mode, or a no-start condition.
The most common cause is a damaged, chafed, or corroded wire in the engine speed output circuit between the TCM and PCM/ECM. A short to ground pulls the signal voltage low.
Internal failure of the TCM can cause a low or missing output signal. This is less common than wiring issues but still plausible.
Corroded or loose pins at the TCM or PCM connectors can create high resistance, dropping the signal voltage.
A defective PCM/ECM that receives the TCM output can misinterpret the signal or fail to provide proper reference voltage. This is rare and should only be considered after thorough wiring and TCM checks.
A weak battery, poor ground connections, or charging system problems can cause low voltage across multiple circuits, including the TCM output.
It depends. If the transmission is in limp mode or shifting erratically, driving may cause further damage. It's best to have the issue diagnosed and repaired promptly.
No. The most common fix is repairing a wiring issue or cleaning a corroded connector. Replacing the TCM is often unnecessary and should only be done after confirming it is faulty.
It means the voltage on the TCM's engine speed output circuit is below the expected range, typically due to a short to ground, open circuit, or a failing component.
Yes, a weak battery or poor ground can cause low system voltage, which may trigger this code. However, it is more likely a circuit-specific issue.