Radiators use fans to cool the engine, which are operated by electric motors. These motors are turned on and off by the PCM (Powertrain Control Module) based on readings from the ECT (Coolant Temperature Sensor). When the ECT sensor reaches a temperature set by the manufacturer, the PCM turns the fans on. When the temperature drops, it turns the fans off.
The Power Control Module or PCM handles the cooling fans through a ground signal. Then, this signal is sent to the cooling fan relay. If the signal circuit is not grounded, the signal voltage is high around the battery voltage. And when it is running or grounded, the voltage is near zero. The OBD2 DTC P0691 is set when the PCM detects low voltage when cooling fan number 1 is off.
To solve the P0691 OBD2 error code faults are as follows:
Locate the fuse related to the system and check it. If you determine that the fuse is blown, you should replace it. When replacing the fuse, make sure the fuse is the correct amperage.
In case the fuse fails after replacing it, it may be due to a short between the fuse and the relay. If the fuse failed after the motor reached operating temperature, the problem would be in the fan motor. To check the condition of the fan, you must rotate the fan blades. If the fan blades do not rotate freely, the fan motor is frozen so you need to replace it.