P0A81: Battery-fan failure uses different voltage checks for on and off commands

P0A81 is the battery cooling-fan unit-failure monitor in this GM battery-controller table. Its feedback-voltage criteria depend on whether the fan is commanded on or off. A voltage without the command state is therefore incomplete diagnostic evidence.

There is more than one failure branch

With the fan commanded on and speed at least 35%, one branch rejects feedback at or above 2.3V or at or below 0.5V. Another branch flags feedback at or above 7V. A further branch evaluates a value between 4V and 7V while the fan is commanded off.

The branches use different failing-sample windows, all within the stated run-mode and low-voltage supply prerequisites. Combining them into one universal threshold or timer would discard how the controller distinguishes operating states.

Separate fan-unit evidence from relay evidence

P0BC1 on the same page is a separate welded-relay monitor. Do not select a relay or fan replacement solely because either code includes the word “fan.” Capture command, reported speed, feedback voltage and controller-performance status together.

The table does not provide a fan connector pinout, airflow test or safe direct-power procedure. A qualified technician needs the vehicle-specific battery-cooling information to investigate the actual fault. Cooling hardware may be near high-voltage equipment, so appropriate isolation and access precautions still apply even though the monitored control signal is low voltage.

Source and applicability

GM 2010 10OBDG10 Hybrid Diagnostics, only the cited controller and monitor. These are historical calibration examples; individual vehicle, engine and hardware applicability is not established. GM PDF, page 239.