What does it mean?
This is a generic powertrain diagnostic trouble code (DTC) applicable to all vehicles newer than 1996. Specific repair steps may vary by brand/model.
The fans used to cool engine radiators are powered by electric motors. These motors are turned on and off by the Powertrain Control Module (PCM) based on readings from the Engine Coolant Temperature (ECT) sensor. In other words, when the ECT reaches a predetermined temperature, the PCM turns on the fans. Then, when the ECT cools to a predetermined temperature, the PCM turns off the fans.
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 (not grounded). The opposite is true when commanded on (grounded); the voltage is somewhere near zero. DTC P0691 is set when the PCM detects low voltage when cooling fan 1 is off.
Symptoms and Severity of the Code
Symptoms of a P0691 code may include:
- Engine overheating
- Check engine light on
The possibility of engine overheating due to inoperative cooling fans is very likely, so caution is needed if driving the vehicle when conditions for setting this DTC are present. It should be repaired as soon as possible.
Possible causes for setting this code are:
- Faulty cooling fan relay 1
- Blown cooling fan 1 fuse
- Faulty PCM (rarely)
Diagnostic and Repair Procedures
Based on my experience, a blown cooling fan fuse is the most common cause of this DTC. Checking the fuse is as simple as locating it, pulling it out, and looking at it. You can usually tell by visual inspection if a fuse is good or not. If the fuse is indeed blown, try replacing it with the correct fuse amperage. If the fuse blows again, the next question would be when did it blow?
If the fuse failed immediately after replacement, there is a direct short somewhere between the fuse and the relay. If it failed when the engine reached operating temperature, the most likely cause is the cooling fan motor itself. With the engine and ignition key off, carefully try to rotate the fan blades by hand. They should spin freely; if not, the fan motor is seized and will need to be replaced. If the fuse is good, the problem is likely with the cooling fan relay. If replacing the relay doesn’t solve the issue, suspect a faulty PCM.
Be cautious when working around components that are not only hot but also moving.