What does it mean?
P1642 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Fan Control Relay 2/3 Control Circuit Malfunction .
Symptoms
- MIL on and, on many vehicles, coolant temperature climbing in traffic or at idle
- Fans never run, or run at full speed constantly, sometimes even after key-off
- A/C blows warm at idle and the module may disable the compressor
- Repeatedly blown cooling fan fuses
- Coolant boiling smell or expansion tank overflow after slow-speed driving
Likely causes
- Failed fan relay, or contacts welded closed inside the relay
- Burnt, spread or loose terminals in the relay socket
- Open, shorted to ground or shorted to battery control wire between module and relay
- Blown fuse or fusible link feeding the relay coil or the fan motors
- Fan motor with high current draw or seized bearings, which cooks relays and terminals
- Water intrusion into the underhood fuse and relay box
- Failed low-side driver inside the control module
How to fix it
- Watch coolant temperature and commanded fan state on live data, then command the fans on and off with the scan tool
- Check the fan fuses and fusible links, then substitute an identical known-good relay
- Back-probe the relay socket for battery feed, ground and the module control signal
- Check continuity from the relay socket to the module pin and test the wire for shorts to ground and to battery voltage
- Power the fan motor directly and measure its current draw; a motor above specification destroys relays
- Repair or replace burnt relay socket terminals and dry out a wet fuse box
- Consider the module driver only after the relay, fan motor and wiring are proven good