OBD-II trouble code
P0691: Cooling Fan 1 Control Circuit Low
The engine computer detected low voltage on the control circuit for the primary cooling fan relay — the circuit is reading near ground when it shouldn't be, pointing to a short to ground or a failed relay. Overheating in traffic is the main risk.
Quick facts
- System
- Powertrain
- Category
- Cooling / Electrical
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $650
- DIY difficulty
- Intermediate DIY
What does P0691 mean?
P0691 is a specific electrical version of a cooling fan control fault. The powertrain control module switches the primary cooling fan (fan 1) through a relay and monitors the voltage on that control circuit. 'Circuit low' means the PCM saw the circuit voltage drop toward ground at a time it didn't expect — typically a short to ground in the wiring, a relay control that's stuck low, or an internal fault in the relay or PCM driver. Its companion code P0692 ('circuit high') describes the opposite electrical condition.
In practice, P0691 most often traces to wiring damage where the harness routes near the radiator, fan shroud, and engine — a chafed wire grounding out is the classic cause — or to a failed relay. A corroded connector or a poor ground reference can also pull the circuit low. Because the PCM reads the fault electrically, the code can be present whether the fan happens to be running or not; the point is that the control circuit isn't behaving the way it should.
The danger is overheating, not driveability. The engine runs fine at highway speed because airflow through the radiator is enough without the fan. The risk shows up in stop-and-go traffic, at idle, with the AC on, or after a hard pull — exactly when the primary fan is supposed to carry the cooling load. If the fault keeps the fan from running, coolant temperature can climb into the danger zone fast, and a single overheating event can warp the cylinder head or fail the head gasket. Treat P0691 as a code that limits where you drive until it's repaired.
Common causes
- Cooling fan 1 control wire shorted to ground (often chafed near the radiator or shroud)
- Failed cooling fan 1 relay (internal short or stuck contacts)
- Corroded or damaged connector at the relay socket or fan motor
- Failed fan motor shorting and pulling the circuit low
- Poor ground or damaged ground reference on the fan circuit
- Melted harness insulation from heat near the engine or exhaust
- Failed PCM fan-control driver (rare, check wiring first)
Symptoms
- Check engine light on with P0691 stored
- Engine temperature climbing in stop-and-go traffic or at idle
- Primary cooling fan not running when it should, or running constantly
- Reduced AC performance at idle and low speed
- Coolant boiling over from the reservoir after a hot shutdown
- Possible blown fan-circuit fuse if the short is dead to ground
- Often no symptoms at highway speed
Diagnostic steps
- 1.Inspect the fan 1 control wiring from the relay toward the fan and PCM, focusing on chafe points near the radiator, shroud, and brackets where a wire can ground out.
- 2.Check the fan 1 relay — swap it with an identical relay from a non-critical circuit, or bench-test the coil and contacts.
- 3.With the relay unplugged, test the control wire for an unintended short to ground; continuity to ground at rest indicates the low-circuit fault.
- 4.Inspect the relay socket and fan motor connector for corrosion, melted plastic, and backed-out terminals.
- 5.Verify the fan motor isn't shorted internally by measuring its resistance and current draw against spec.
- 6.Confirm the circuit ground and reference are solid before suspecting the PCM.
- 7.Use a scan tool to command the fan and watch the control circuit respond; a fault that resets immediately confirms the short.
Repair cost
$50 – $650
Repairing a chafed or shorted control wire runs $100-$300 depending on access. A cooling fan relay is $20-$80 in parts, so a relay-only fix is about $80-$150 at a shop. If the fan motor is shorted or the full fan-and-shroud assembly needs replacing, expect $300-$650. Compared with the $1,500-$3,000 a single overheating event can cost in head gasket damage, repairing P0691 promptly is well worth it.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with cooling fan motor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.