Possible causes

– Faulty Cooling Fan Relay
– Cooling Fan Relay harness is open or shorted
– Cooling Fan Relay circuit poor electrical component

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Cooling Fan not working

P1642 GMC Description

Ignition voltage is supplied to the Cooling Fan relay coil. The Powertrain Control Module (PCM) controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line which is monitored by the PCM. When the PCM is commanding a component ON, the voltage of the control circuit should be lo (near 0 volts). When the PCM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be high (near battery voltage). If the fault detection circuit senses a voltage other than what is expected, the fault line status will change causing the code to set.

The relay is used to control the high current flow to the cooling fan motors. This allows the PCM driver to only have to handle the relatively low current used by the raly.