What causes a P0691 code?

P0691 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a P0691 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P0691 active.

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

OBD-II

P0691

Cooling Fan 1 Control Circuit Low Voltage

What it means

The Engine Control Module (ECM) detects an electrical short to ground or an open circuit in the radiator cooling fan control circuit. This prevents the fan from engaging properly, risk overheating the engine.