Cooling

P0691 — Cooling Fan 1 Relay Control Circuit Low

Severity: medium Drive with care, to a shop

What P0691 means

The engine control module switches the cooling fan on by sending a control signal to the fan relay coil. With P0691, the voltage in that control circuit stays lower than expected, typically from a short to ground or a faulty driver inside the module. The fan may then run all the time or never turn on.

Symptoms

  • Engine overheating in traffic or at idle
  • Cooling fan never turns on
  • Fan runs constantly after key on
  • Check Engine light on
  • Temperature gauge climbing into the red
  • Weak A/C performance when stopped

Possible causes

  1. Short to ground in the fan relay control wire
  2. Failed fan relay (stuck contacts or bad coil)
  3. Chafed or damaged wiring harness
  4. Faulty output driver inside the ECM
  5. Shorted cooling fan motor
  6. Corroded or wet connectors at the relay or fuse box

How to diagnose

  1. Scan for codes, review freeze frame data, then clear the code and see if it returns
  2. Inspect the fuse and relay box and connectors for moisture, burning and corrosion
  3. Swap the fan relay with a known good identical one and retest
  4. Command the fan on with a scan tool and see if it responds
  5. Unplug the relay and test the control wire for a short to ground with a multimeter
  6. Inspect the harness from the ECM to the relay, especially where it bends or touches the body
  7. Check the fan motor for free rotation and normal current draw
  8. If wiring is good, test the ECM output and repair or replace the module if needed

Common fixes

  • Replace the faulty fan relay
  • Repair the shorted wire and restore its insulation
  • Replace corroded connectors or terminals
  • Replace the fan motor if it is causing the short
  • Repair or replace the ECM if the driver has failed

Questions and answers

What does P0691 mean?

The ECM detected low voltage in the control circuit of cooling fan relay 1. The usual causes are a short to ground, a bad relay or damaged wiring.

Can I drive with P0691?

Only briefly and with close attention to engine temperature, and avoid traffic. If the fan does not run, the engine can overheat, so repair it soon.

Will replacing the relay fix it?

Often yes, and it is the cheapest first test. If the code returns, look for a wiring short, a bad fan motor or an ECM fault.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.