P0691 Code: Fan 1 Control Circuit Low

What the P0691 code means

The computer turns the radiator fan on through relays or a control module when coolant temperature climbs or the air conditioning is running.

The computer watches the control circuit from its own driver transistor. The P0691 code tells you the voltage in that circuit is low, which means a short to ground or a loss of supply. Before you replace any part, check the fuse, the relay, the connector and the wiring, because those are the most common causes.

If the fan does not work, the engine overheats in slow traffic and the A/C blows weak at a standstill. Keep an eye on the temperature gauge until the repair is done.

To confirm the fault, unplug the fan with the key on and watch what happens to the reading or the code. If the value jumps to the opposite extreme, the wiring is fine and the fan is the part pulling the signal to ground. If nothing changes, look for the short in the wire itself, starting where the harness rubs against the engine, the chassis or the exhaust.

P0691 code symptoms

• Check engine light or another warning light on.
• Overheating in slow traffic.
• Temperature climbing in traffic or when stopped.
• A/C that cools poorly at a standstill.

Likely causes of the P0691 code

1. Control wire shorted to ground. The fan may stay on all the time, or the computer cuts the command to protect itself.
2. Interrupted supply: a blown fuse, a faulty relay or a cut wire.
3. Shorted fan motor or relay. The resistance is far lower than specified.
4. Connector with moisture, which creates a path to ground.

How to diagnose the P0691 code step by step

1. Command the fan with the scan tool and check whether it responds.
2. Check the fuse and the relay. Measure voltage on both sides of the fuse, and swap the relay with an identical one.
3. Measure the supply at the connector. You should see battery voltage with the key on or when the fan is commanded.
4. Measure the fan resistance and compare it with the manual. Also check the insulation to ground.
5. Check the control wire for continuity all the way to the computer and for any short circuits.

How to fix the P0691 code

• Repair the connector, the wiring or the power supply.
• Replace cooling fan 1 if it does not respond when it has correct power and a correct control signal.
• Clear the P0691 code and verify the repair with a test drive and a check that the fan turns on.

Technical reference data

• Typical current draw: 10 to 30 amps.
• Reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

On many vehicles the two-speed fan uses resistors or a PWM module, so the circuit layout varies from one model to the next. Look up the wiring diagram for your vehicle before you probe anything.

Frequently asked questions about P0691