Faulty fan 1 relay (stuck contacts or failed coil); Damaged, chafed or corroded wiring in the relay control circuit (open circuit or short to ground or voltage); Poor connection at the relay socket or fuse and relay box terminals; Blown fuse or a fault in the relay power supply feed; Faulty cooling fan motor drawing excess current and overloading the circuit; Failed driver circuit inside the powertrain control module.

P1651

Fan 1 Relay Control Circuit.

GMC
Powertrain
Manufacturer code
High severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
6: AreaComputer and output circuits
51: FaultThe specific fault in that area

What it means

P1651 on a GMC means the powertrain control module has detected a fault in the control circuit for the number 1 cooling fan relay. The module commands the relay on and off and monitors the circuit voltage. When the voltage does not match what it commanded, such as a short, an open circuit or a relay that does not respond, it sets the code.

Symptoms

Engine cooling fan not running, or running all the time
Engine running hot or overheating in traffic or at idle
Temperature gauge climbing when stationary
Air conditioning performing poorly at idle
Check engine light on
Fan running on after the engine is switched off

Possible causes

Faulty fan 1 relay (stuck contacts or failed coil)likely
Damaged, chafed or corroded wiring in the relay control circuit (open circuit or short to ground or voltage)likely
Poor connection at the relay socket or fuse and relay box terminalslikely
Blown fuse or a fault in the relay power supply feedpossible
Faulty cooling fan motor drawing excess current and overloading the circuitpossible
Failed driver circuit inside the powertrain control modulerare

How to diagnose

Read the stored codes and freeze frame data, noting coolant temperature and whether other cooling or fan codes are present
Check the relevant fuses and visually inspect the relay box, relay socket and wiring for heat damage, corrosion or water ingress
Using the scan tool's bidirectional controls, command the fan 1 relay on and off and listen for the relay clicking and the fan responding
Swap the fan 1 relay with a known good identical relay, or test the relay coil and contacts on the bench
Check for battery voltage and a good earth at the relay socket with the circuit powered, following the wiring diagram for the specific model and engine
Test the control wire between the relay and the PCM for continuity, and for shorts to ground or voltage, with the PCM connector disconnected
Check fan motor current draw and the fan motor connector for signs of overheating
If the wiring and relay are proven good, check the PCM connector pins and consider PCM driver failure

How to fix

Replace the faulty fan 1 relay with a correct specification part
Repair or replace damaged wiring and clean or renew corroded connector terminals
Replace the blown fuse after finding and correcting the cause of the overload
Replace a failed cooling fan motor or fan assembly
Repair or replace the PCM only after all other causes are eliminated, and have it programmed as required

Can I keep driving?

High severity

Avoid driving until the fan is confirmed working, as a failed fan can quickly cause overheating and serious engine damage. If you must move the car, keep trips short, watch the temperature gauge and stop immediately if it rises.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.