P0494: Measured cooling-fan speed falls short of a modeled response

P0494 is a cooling-fan underspeed diagnostic. The cited GM implementation applies to LIN-bus electric PWM fans with internal or external controllers. It compares measured fan speed with an expected speed model rather than demanding that speed instantly equal the command.

The model allows normal response delay

Expected speed accounts for startup, ramp-up and transport delays, command rate limiting and supply-voltage compensation. Therefore, a brief lag after a speed-command increase is not automatically a failure. The relevant quantity is the measured-versus-modeled error against the calibration's low-speed limit.

The test uses sixteen failures out of twenty samples at one second per sample. It requires the fan commanded on, battery voltage in range, enabled LIN operation and fan speed at least 690 rpm. Lost communication and several fan status/range faults inhibit it.

Check data validity before condemning the fan

Record requested, expected and measured speed together with voltage and LIN status. The source also says the first activation can hold the fan commanded on long enough for the diagnostic to reach a decision. That sustained command can be part of the test rather than proof of a separate control fault.

Verify the applicable low-speed calibration and a completed test after diagnosis. The code does not alone distinguish a motor, controller or physical-load problem. Follow approved cooling-fan procedures; a rotating or unexpectedly starting fan is not suitable for hands-on probing.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 306, page 307. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.