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DTC codes / GM codes / P1491

P1491 · GMRadiator Fan Control Relay Circuit Open Or Shorted

P1491 on a GM vehicle means the powertrain control module (PCM) has detected an open or shorted circuit in the radiator fan control relay circuit. The PCM commands the relay on or off and monitors the circuit for expected voltage or continuity, so this code points to a wiring, relay, or connection fault rather than a mechanical cooling problem.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • The service engine soon light is illuminated, often with no other symptoms.
  • The engine coolant temperature gauge may climb higher than normal in traffic or at idle because the fan does not run.
  • The cooling fan may run continuously even with the engine cold or the key off, depending on whether the circuit is open or shorted.
  • The air conditioning may perform poorly or stop cooling when the vehicle is stationary because the condenser fan is not commanded on.
  • You may hear the fan relay click repeatedly or not at all when the engine warms up.
  • In some cases the engine may enter reduced power mode or set companion codes if the PCM sees an unexpected fan circuit condition.

Common causes, most likely first

  1. A failed radiator fan control relay, often due to burned contacts or a cracked solder joint inside the relay.
  2. Corroded, loose, or pushed-back terminals at the relay socket or fan connector.
  3. A broken, chafed, or shorted wire in the fan control circuit, commonly near the fan shroud or under the airbox.
  4. A seized or shorted radiator fan motor that draws excessive current and damages the relay or circuit.
  5. A poor ground connection for the fan motor or relay, especially on rust-prone chassis points.
  6. A faulty powertrain control module (PCM) driver for the fan relay, though this is less common than wiring or relay faults.
  7. Aftermarket or incorrectly installed fan components that do not match the factory circuit design.

How to diagnose and fix it

  1. Clear the code and verify it returns, then check the radiator fan relay by swapping it with an identical known-good relay from the fuse box and retesting.
  2. Inspect the relay socket and fan connector terminals for corrosion, looseness, or heat damage, and repair any damaged terminals.
  3. Use a digital multimeter to check for battery voltage at the relay control circuit with the key on and the fan commanded on, and check for a good ground on the ground side.
  4. Disconnect the fan motor and measure its resistance and current draw; compare to factory specifications and replace the fan assembly if it is seized or out of spec.
  5. Perform a voltage drop test on the power and ground circuits to the fan motor while the fan is commanded on to find hidden resistance in the wiring.
  6. Trace the fan control wire from the PCM to the relay for continuity and shorts to ground or power, repairing any chafed or broken wires.
  7. If all wiring, relay, and fan motor tests pass, follow the factory diagnostic flow for PCM driver testing before condemning the PCM.

Fan relay circuit faults that fool technicians

  • A fan motor that spins freely by hand can still have an internal short that only appears when it gets hot, so always check current draw rather than just resistance.
  • The relay socket often looks fine from the top but has spread terminals underneath that lose tension after years of heat cycling, causing an intermittent open circuit.
  • A shorted fan motor can damage the new relay you just installed, so test the motor circuit before replacing the relay a second time.
  • Some GM vehicles use a fan control module instead of a simple relay, and P1491 can set from a module communication fault even when the relay itself is good.
  • The PCM may command the fan on for air conditioning pressure or after-boil protection even when the engine is not hot, so a fan that runs with the key off is not always a stuck relay.

Can I keep driving with P1491?

You can usually drive the vehicle short distances if the temperature gauge stays in the normal range, but avoid heavy traffic, long idles, and towing because the fan may not run and the engine can overheat. If the temperature gauge climbs above normal or the air conditioning stops cooling at a stop, pull over and let the engine cool before continuing.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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