P0D66: Battery cooling-fan enable stays high when off

P0D66 means hybrid/EV battery pack cooling-fan enable circuit high. The 2018 GM implementation evaluates an apparent short to power in the output-enable circuit with cooling-fan enable false.

A command-feedback mismatch

The documented high band is 5.27–19.7 V. System voltage must exceed 10.2 V, and the monitor requires 16 failed samples among 20 at 250-millisecond intervals. The listed evaluation is five seconds with Type B, two-trip reporting.

These control-circuit voltages do not describe traction-battery voltage. Nor does the code directly confirm that the fan is mechanically overspeeding or continuously rotating. Its evidence is the enable output's electrical response during an off command.

Make verification match the failure state

Preserve the false enable command together with feedback and supply information. A high value during an intentional fan-on command is not interchangeable with the condition that sets this fault. The vehicle-specific diagnostic must determine whether the output path, load or controller is responsible.

After repair, confirm that enable feedback responds appropriately when the command goes off and that the prescribed fan operation tests complete. A fan that runs successfully when requested may still have an off-state circuit fault, so that observation alone is insufficient. This table is not a pin-by-pin test procedure and provides no reason to bridge or power the enable circuit externally. Follow the applicable hybrid-system service precautions.

Source and applicability

This explanation follows the documented GM monitor, not a verified application to every vehicle with this code. Confirm the vehicle, reporting module and calibration in current service information. GM diagnostic table, physical page 953.