The vehicle engine has a variable speed cooling fan, which is controlled by the ECM (Engine Control Module) employing a pulse width controlled signal that is sent to the Cooling Fan Speed Control Module. The latter module has changes in the voltage drop across the fan motor according to the pulse width modulated signal.
The cooling fan speed depends on certain factors and can be adjusted from 10 to 90% of the duty cycle. The 90% is considered the high fan speed. When multiple cooling speed requests are received, the ECM will use the highest speed for all of them. During proper fan operation, the control circuit voltage will be low at Power On and close to battery voltage when the fan is Off.
The P1481 OBDII diagnostic code setting occurs when the ECM detects that the cooling fan signal has been lost, either intermittently or completely.