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DTC codes / GM codes / P1490

P1490 · GMLow Speed Fan CTRL Relay Circuit Open Or Shorted

This code means the engine control module (ECM) commanded the low-speed cooling fan relay on, but the relay control circuit did not respond as expected, so the ECM flagged the circuit as open or shorted. The ECM monitors the voltage on the relay control wire and sets P1490 when that voltage stays outside the expected range for the commanded state, which usually leaves the low-speed fan inoperative.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • The check engine light is on, and often there are no other symptoms besides the warning light.
  • The low-speed cooling fan does not run when the engine is warm or when the air conditioning is switched on.
  • Engine temperature climbs higher than normal in traffic or during long idle periods because only the high-speed fan is available.
  • The air conditioning may blow warm at idle because the condenser is not getting enough airflow without the low-speed fan.
  • In some cases the low-speed fan runs continuously with the key on because the control circuit is shorted to ground.
  • A cooling fan relay that clicks repeatedly or feels hot to the touch after the engine is shut off.

Common causes, most likely first

  1. A failed low-speed cooling fan relay, usually from burned contacts or a corroded internal coil.
  2. An open or shorted wire in the relay control circuit between the ECM and the relay, often from chafing or rodent damage.
  3. A corroded or loose relay socket terminal or connector pin at the underhood fuse block.
  4. A failed cooling fan motor that draws excessive current and damages the relay or control circuit.
  5. A poor ground connection at the relay or fan motor mounting point.
  6. A faulty ECM driver on the fan control output, which is less common but does occur.
  7. Water intrusion into the underhood fuse block or relay box causing corrosion on the control circuit.

How to diagnose and fix it

  1. Scan all modules for stored codes and record freeze-frame data to confirm P1490 is current and not a history code from a past repair.
  2. Inspect the low-speed fan relay, its socket, and the underhood fuse block for corrosion, loose pins, or a relay that fails a click test when commanded on with a scan tool.
  3. Check the fan motor by applying battery voltage directly to the motor terminals to confirm it spins freely and does not draw excessive current.
  4. Measure resistance and voltage drop on the relay control wire from the ECM connector to the relay socket, looking for opens, shorts to ground, or shorts to voltage.
  5. Verify the relay ground circuit has less than one ohm of resistance to the battery negative terminal.
  6. Command the low-speed fan on with a scan tool and watch the control circuit voltage at the ECM to confirm the driver is functioning.
  7. Replace the failed relay, repair the damaged wire, or replace the fan motor as indicated, then clear codes and verify the low-speed fan runs when commanded.

What the low-speed fan circuit hides

  • A fan motor that still spins by hand can draw enough current to damage the relay contacts without blowing a fuse, so always check motor current draw before replacing a relay.
  • The low-speed fan relay is often shared with other functions in the underhood fuse block, and corrosion from a leaking windshield cowl or a clogged sunroof drain can attack that block and set this code.
  • A scan tool that can command the fan on is more useful than a multimeter alone, because the ECM may set P1490 only when the commanded state and the actual circuit voltage disagree.
  • Some GM vehicles use a fan control module instead of a simple relay, and a failed module can set a relay circuit code even though the relay itself tests good.
  • The low-speed fan may not run during a key-on engine-off test on certain models, so verify the command strategy in the service information before condemning the circuit.

Can I keep driving with P1490?

You can usually drive the vehicle to a repair shop, but avoid heavy traffic, long idle periods, and towing until the fan is fixed because the engine may overheat without the low-speed fan. If the temperature gauge climbs into the red or the air conditioning stops cooling at idle, pull over and let the engine cool.

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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