OBD-II code P0691 indicates that the powertrain control module (PCM) has detected a low voltage condition in the control circuit for cooling fan 1. This typically means the circuit is shorted to ground, open, or has high resistance, preventing the fan from operating correctly. The code is set when the PCM commands the fan on and the feedback voltage is below the expected threshold.
The relay contacts may be welded, pitted, or the coil may be open or shorted, preventing proper circuit control and causing a low voltage reading on the control side.
A short to ground, open circuit, or high resistance in the wiring between the PCM and the fan relay or fan module can cause the control circuit voltage to drop below the expected level.
A blown fuse interrupts power to the relay coil or fan control module, resulting in no voltage on the control circuit when the PCM expects it to be active.
If the fan assembly includes an electronic control module, an internal failure can pull the control circuit low or fail to provide proper feedback, triggering the code.
An internal driver failure in the PCM is a rare cause, but can occur if the control circuit is shorted to power or ground, damaging the output transistor. This should only be considered after all other possibilities are ruled out.
It is not recommended, as the engine may overheat due to the cooling fan not operating properly. Overheating can cause severe engine damage. If you must drive, monitor the temperature gauge closely and avoid idling or heavy traffic.
Yes, if the fan motor is part of a module-controlled assembly or has an internal short that affects the control circuit. However, the code specifically points to the control circuit, so a faulty relay, wiring, or fuse is more common.
PCM replacement is rarely the correct fix. Many shops misdiagnose this code and replace the PCM unnecessarily. The cause is usually a simple electrical fault like a bad relay, blown fuse, or damaged wiring. Always rule out these common issues first.
While unlikely, a severely low system voltage could potentially cause erratic circuit behavior and set this code. Check battery and alternator health if no other cause is found, but focus on the fan control circuit first.