Oki OKIFAX 5750 Service Manual page 837

To detect fan errors, the fan sensor (FANALM-P) is monitored in the full speed mode. The fan sensor is not monitored for 3 seconds after the start of full speed
operation taking into account the fan sensor output determination time.
The fan sensor is not monitored when the fan is rotating at 1/2 or 1/4 speed or it is at halt.
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