Fan 1 Control Circuit Low
The engine cooling fan isn't responding properly to the ECU's commands—it's like pressing the gas pedal but the engine won't rev because the signal isn't reaching the fuel injector. The ECU detects abnormally low voltage or current in the fan control circuit, meaning the fan motor or its wiring has a problem.
The ECU sends a ground signal to activate the cooling fan relay or fan motor directly through a control circuit. It monitors the voltage drop and current draw across this circuit. When voltage stays too high (fan doesn't pull current, indicating an open or disconnected circuit), the ECU sets P0691.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Fan Control Circuit Voltage | 0.5–2V (when activated) | >4.5V or no current draw detected |
| Fan Control Circuit Resistance | <5 ohms at relay coil | >50 ohms or open circuit |
Code P0691 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0691 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.