P0691
PowertrainCooling Fan 1 Relay Control Circuit Low
The ECU controls the cooling fan relay by sending it a signal to switch on and off, and it watches the voltage on that control wire to make sure the relay is responding. When it sees the voltage drop lower than it should (a short to earth or a dead circuit), it logs P0691. For you, that means the computer thinks it can't reliably turn the cooling fan on, so the fan might not kick in when the engine needs it.
ⓘ Information only. This page provides general educational information about fault code P0691. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0691 mean?
P0691 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cooling Fan 1 Relay Control Circuit Low.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Temperature gauge creeping up when you're stuck in traffic or sitting at idle, then dropping again once you're moving
- • Check engine light on
- • Fan never spinning up even when the engine's hot, which is the one that gets people stranded
- • Air con blowing lukewarm at low speeds because the same fan often helps cool the condenser
- • Fan running flat out and refusing to switch off, even after you've pulled the key out
- • A clicking or buzzing from the relay area under the bonnet
Possible causes
Causes commonly associated with P0691, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Blown cooling fan fuse, usually because the fan motor has drawn too much current or shorted. Check this first because it's the cheapest
- 2. Failed fan relay that's no longer switching cleanly. These get hammered with heat cycles and are a known weak point
- 3. Corroded or chafed wiring in the control circuit, very common where the loom runs near the fan and gets road salt and water thrown at it
- 4. A tired fan motor pulling excessive current, which can take the fuse or relay out with it
- 5. Faulty coolant temperature sensor feeding the ECU duff readings, less common but it does happen
- 6. PCM fault, which is rare and only worth considering once everything upstream checks out clean
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the codes and check the freeze-frame data for what the engine was doing when it logged. Note any other fan-related codes sitting alongside it
- 2. Check the cooling fan fuse for a blown element or heat discolouration. A blown fuse points you straight at a short or a binding motor
- 3. Swap the fan relay with an identical one from another circuit and see if the fault moves with it. Cheap, quick, and rules the relay in or out
- 4. Back-probe the control wire at the relay with a multimeter while commanding the fan on with a scan tool. Low or no voltage here tells you whether it's the ECU side or the relay side
- 5. Go through the wiring and connectors with a torch, looking for green corrosion, melted insulation, or pins backing out of the plug
- 6. Spin the fan blades by hand with the ignition off. If they're stiff or grind, the motor's on the way out and it'll keep eating fuses
Common questions about P0691
If I clear the code, will it stay gone? +
If it's a marginal connection or a one-off, it might clear and not come straight back. But if there's a blown fuse, a dead relay, or chafed wiring, P0691 will return as soon as the ECU tries to command the fan again. Clearing it without finding the cause just hides the problem until the next hot day in traffic, which is exactly when you don't want the fan failing.
What am I risking if I just leave it? +
The big risk is overheating. If the fan can't switch on, the engine cools fine while you're moving at speed because of airflow through the radiator, but the moment you're crawling in traffic or idling, the temperature climbs fast. Let it go far enough and you're looking at a warped head, a blown head gasket, or a cooked engine. That's a few hundred pounds of fan work turning into a few thousand.
How quickly do I need to sort this? +
Soon. You can usually still drive the car, but keep one eye on the temperature gauge and don't sit in stationary traffic. On a cool morning doing motorway miles you're at lower risk. Stop-start town driving or a hot day is where it bites. Treat it as a this-week job, not a leave-it-till-the-service job.
Is it the relay itself or the wiring causing this? +
P0691 is specifically a low-voltage fault on the control circuit, so it's more often wiring, a connector, or the relay than the fan motor. Corrosion and chafed insulation where the loom runs near the fan are common culprits on higher-mileage cars. Swap the relay first since it's the easy win, but if that doesn't fix it, get the multimeter on the control wire and follow it back to find where the voltage is being dragged down.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →