P0691 Cooling Fan 1 Relay Control Circuit Low

Description and meaning of DTC p0691

This is a generic powertrain diagnostic trouble code (DTC) applicable to all 1996-newer vehicles. Specific repair steps can vary from make/model. The fans that are used to cool the engines radiator are run by electric motors. These motors are turned on and off by the powertrain control module (PCM) according to the readings from the engine coolant temperature sensor (ECT). In other words when the ECT reaches a predetermined temperature the PCM turns the fans on. Then when the ECT cools to a predetermined temperature the PCM turns the fans off. The PCM controls the cooling fans by sending a ground signal to the cooling fan relay. The voltage on the signal circuit is high, around battery voltage, when not commanded on (not grounded). The opposite is true when commanded on (grounded), the voltage is somewhere near zero. DTC P0691 is set when the PCM senses the voltage low when the cooling fan 1 is off.

p0691 diagnostic trouble code symptoms

Symptoms of a P0691 code may include: Engine overheating Check engine light on The possibility of the engine overheating due to non-operational cooling fans is very probable so caution should be used if driving the vehicle when the conditions to set this DTC are present. It should be repaired as soon as possible.

DTC p0691 - possible causes

Potential causes for this code to set are: Faulty cooling fan relay 1 Blown cooling fan 1 fuse Faulty PCM (rarely)

How to fix OBD-II diagnostic trouble code p0691

In my experience a blown cooling fan fuse is the most common cause of this DTC. Checking the fuse is a simple as locating, pulling it and looking at it. You can generally tell by visual inspection if a fuse is good or not. If the fuse is in fact blown, try replacing it with the correct amperage of fuse. If the fuse blows again the next question would be when did it blow? If the fuse failed immediately upon replacing it then there is a direct short somewhere between the fuse and the relay. If it failed when the engine reached operating temperature then the most likely cause is the cooling fan motor itself. With the engine and the ignition key off, carefully try and spin the fan blades by hand. They should spin freely, if it does not then the fan motor is frozen and will need replacement. If the fuse is good then the problem is most likely the cooling fan relay. If replacing the relay does not solve the problem suspect a faulty PCM. Be sure to use caution when working around components that are not only hot but also moving.

More OBD-II diagnostic trouble codes (DTC)