P0691 code definition
The P0691 code appears when the powertrain control module (PCM) has detected an inappropriate level of voltage in the electric cooling fan's control circuit. The level can be too high or too low.
What the P0691 code means
P0691 is an OBD-II generic code triggered when voltage varies by more than 10% of the vehicle manufacturer's explicit reference voltage.
What causes the P0691 code?
The code may have several reasons for appearing:
- A faulty cooling fan relay
- A defective cooling fan motor
- Electrical connectors that have corroded or become loose
- A defective engine coolant temperature sensor
- Shorted or open electrical wiring that has caused a fuse to blow
What are the symptoms of the P0691 code?
Typically, the P0691 code has almost no symptoms except the illumination of the check engine light, though an engine may overheat because of this issue and any air conditioning systems may operate less effectively.
How does a mechanic diagnose the P0691 code?
P0691 is diagnosed with an OBD-II scanner, digital ohm meter and an infrared thermometer can be very useful. Diagnosing this can be somewhat challenging without knowing the manufacturer's specs regarding cooling fans for the engine and the air conditioner. The mechanic or technician will then use a scanner to activate the engine fan and check voltage and grounding of the fan motor. No voltage means the system fuses must then be tested, and if the fuses are acceptable, the fan relay must be checked. The voltage at the connector must be within manufacturer specs.
The engine temperature readings must then be tested and if not within the manufacturer specs, it is likely a faulty engine coolant temperature sensor. Typically, this series of tests will reveal the source of the code, and the technician will then repair any open or shorted circuitry as detected by the diagnostics before rechecking with the scanner.
Common mistakes when diagnosing the P0691 code
Any mistakes in diagnosis tend to come from the replacing of cooling fan motors prior to running the multiple steps of diagnosis. There are primary fans and some secondary fans, and the mechanic has to run a full test before simply replacing the primary fan motor.
How serious is the P0691 code?
The P0691 code does not prevent the car from running, but some vehicles can overheat at idle when this problem persists. Poorly operating air conditioning is also undesirable and is a common consequence of this code.
What repairs can fix the P0691 code?
To address the P0691 code, the following repairs are typically used:
- The mechanic will verify the code with a scanner and then obtain the wiring schematic for the vehicle.
- The PCM wiring and connectors are assessed and repaired or updated as needed, and the diagnostic is repeated, with freeze frame data noted.
- Run diagnostics in proper order, including testing the engine cooling fan and its voltage and grounding. Then the system fuses, and then the cooling fan relay are tested and any circuitry repaired or replaced.
- Engine temperature readings are tested and should be within manufacturer's specs, and an engine coolant temperature sensor is replaced if the numbers are flawed.
Additional comments for consideration regarding the P0691 code
PCM issues can be an issue, but this is a rarity. You may also have to run diagnostics identical to those above on any secondary cooling fan systems and their associated circuitry. Controlled by dual systems, they are most often meant to provide air to the air conditioning systems and are run via sensors relating to the air conditioning and not the engine coolant sensors. You may need to track the system that activates a secondary or cooling fan to determine the source of the problem.
Need help with a P0691 code?
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