Error Code P0691 is defined as Cooling Fan 1 Relay Control Circuit Low. This code is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. Specifications on the description, diagnosis, and repairs, of course, may vary from one make and model to another.
Electric motors run the fan to cool the radiator in the engine. These motors are turned on and off by the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), depending on the readings it gets from the ECT (engine coolant sensor). This means, when the ECT reaches a set temperature, the PCM will turn the fans on. When the ECT cools to a predetermined temperature, then the PCM turns off the fans.
The PCM controls the cooling fans by sending a ground signal to the cooling fan relay. When not commanded on (not grounded), then the voltage on the signal circuit is high, around battery voltage. The opposite is true when commanded on (grounded), the voltage is somewhere near zero. Error Code P0691 is set when the PCM senses the voltage low when the cooling fan 1 is off.
Since this code affects the fan, then engine overheating is the most common symptom. Non-operational cooling fan can cause the engine to overheat when driving. Thus, it’s important to be cautious when driving a vehicle with this code. Therefore, it should be addressed as soon as possible.
This code can have several reasons for appearing, such as:
- Defective cooling fan relay 1
- Blown cooling fan 1 fuse
- Defective cooling fan motor
- Corroded or loose electrical connectors
- Defective engine coolant temperature sensor
- Open or short in electrical wiring that caused the fuse to blow
How to Check
A blown fuse is the most common cause of this code. Thus, it should be the first thing to check. Look for the fuse, pull it out and check it. You can generally tell if the fuse has been damaged or not.
If the fuse has been blown out, then replace it with a fuse with the right amperage. If the new fuse blows out again, then it’s time to check when did it blew.
If the fuse immediately failed after replacing it, then there must be a direct short somewhere between the fuse and the relay. If it failed when the engine reached the right operating temperature, then the common cause would be the cooling fan motor itself.
With engine and ignition key off, try to spin the fan blades by hand (be careful though) to check if the fan can spin freely. If it couldn’t spin freely, then the motor is frozen and must be replaced.
If the fuse is good, then the problem must be in the cooling fan relay. If replacing the relay doesn’t solve the problem, then you can suspect a defective PCM.
When working around components that move or are hot, be sure to use caution.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of the blown fuse
- Repair or replacement of the open or shorted wiring between the PCM and the fuse
- Repair or replacement of cooling fan relay
- Repair or replacement of cooling fan motor
This code may not prevent the vehicle from running, but overheating is an obvious risk.
The most common mistake in diagnosing this code is replacing the cooling fan motor before running multiple tests for proper diagnosis. There are primary fans and secondary fans, and they must be all checked before replacing the primary fan motor.