P0691
Fan 1 Control Circuit Low
Powertrain Engine Cooling Fan Control Circuit 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Engine running hot or overheating
Fan not spinning when engine temperature rises
Check Engine Light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Fan relay
Locate the cooling fan relay in the engine bay fuse box, pull it out, and install a new OEM or equivalent relay; test the fan operation.
2
Fan control wiring and connectors
Inspect the wiring harness between the ECU and fan relay/motor for corrosion, loose connectors, or damaged insulation and repair or reseat as needed.
3
Cooling fan motor
If relay and wiring are good, test the fan motor with direct 12V power; if it doesn't spin, replace the fan motor assembly.