Technical Description of OBD-II Trouble Code
The control circuit for Cooling Fan 1 Relay is experiencing low voltage.
What does that mean?
This is a generic powertrain diagnostic trouble code (DTC) that applies to all vehicles manufactured from 1996 onwards. The specific repair steps may vary depending on the make and model of the vehicle.
The electric motors used to cool the engine’s radiator are responsible for running the fans. The powertrain control module (PCM) turns these motors on and off based on readings from the engine coolant temperature sensor (ECT). In other words, when the ECT reaches a certain temperature, the PCM activates the fans. When the ECT cools down 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. When the fans are not commanded on, the voltage on the signal circuit is high, close to the battery voltage. Conversely, when the fans are commanded on, the voltage is close to zero. DTC P0691 is triggered when the PCM detects a low voltage when the cooling fan 1 is off.
Symptoms & Severity of Code
Symptoms of a P0691 code may include:
If the conditions that trigger this DTC are present, there is a high possibility of the engine overheating due to non-operational cooling fans. Therefore, caution should be exercised when driving the vehicle. It is recommended to have the issue repaired as soon as possible.
Causes
Possible causes for this code to be triggered are:
Diagnostic and Repair Procedures
In 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 visually inspecting it. You can usually tell if a fuse is good or not by its appearance. If the fuse is indeed blown, try replacing it with the correct amperage fuse. If the fuse blows again, the next question to consider is when it blew.
If the fuse failed immediately after replacement, there may be a direct short circuit between the fuse and the relay. If it failed when the engine reached operating temperature, the most likely cause is a frozen cooling fan motor. With the engine and ignition key off, try spinning the fan blades by hand. They should rotate freely. If they do not, the fan motor is frozen and needs to be replaced. If the fuse is intact, the problem is most likely the cooling fan relay. If replacing the relay does not resolve the issue, a faulty PCM may be suspected.
Always exercise caution when working around components that are not only hot but also in motion.
