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P codepowertrainMedium severity

P0691Fan 1 Control Circuit Low

The control circuit for cooling fan 1 is reading lower voltage than expected, usually from a short to ground in the fan relay, wiring, or control module.

Definition

The PCM switches cooling fan 1 through a relay or an electronic fan control module and monitors the control line for feedback. P0691 sets when the circuit voltage is below the expected value for the commanded state, typically indicating a short to ground or excessive current draw. The fan may run constantly, never run, or run at the wrong speed, which can cause overheating in traffic, poor A/C performance at idle, and an illuminated MIL.

Common causes

  • Fan control wiring chafed and shorted to ground near the fan shroud or harness routing points
  • Shorted fan relay coil or failed relay
  • Cooling fan motor with dragging bearings or shorted windings pulling excessive current
  • Corroded or water-intruded connector at the fan assembly or relay
  • Failed fan control module (on vehicles with variable-speed fans)
  • Faulty PCM low-side driver

Common misdiagnoses

  • Fan motor replaced when the relay or wiring is shorted
  • Thermostat or coolant temperature sensor replaced for the resulting overheat complaint
  • PCM replaced without checking the circuit for a short to ground

Troubleshooting steps

  1. 1.Command the fan with a bidirectional scan tool

    Verify whether fan 1 responds to on/off or duty-cycle commands and note the state in which the code sets.

  2. 2.Measure control-circuit voltage in both commanded states

    With the driver off, most relay-controlled circuits should show near battery voltage on the control side; a constant near-0 V reading indicates a short to ground.

  3. 3.Test the fan relay

    Check coil resistance (roughly 50–120 ohms) and substitute a known-good relay to rule out a shorted coil.

  4. 4.Check fan motor current draw

    Clamp the fan feed and compare running amperage to spec — a dragging motor can pull the control output low and damage the driver.

  5. 5.Wiggle-test the harness

    Flex the wiring between the PCM, relay box, and fan connector while watching live data for the fault to appear, paying attention to chafe points on the shroud.

  6. 6.Verify the PCM driver last

    Only after the wiring, relay, and motor test good should the PCM output driver be suspected; confirm with a load test at the PCM connector.

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