The PCM controls the fans based on coolant temperature sensor input and A/C refrigerant pressure sensor input. The PCM then activates the cooling fan relays (low and high speed) located in the under-hood fuse/relay box. If you unplug the A/C pressure sensor, the PCM assumes a fault and commands the fans on as a failsafe — which explains why they run in that condition.
Since the fans run when forced we know the fan motors are good and the wiring from relay to fan motors is good. Issue is either in control (relay/PCM/sensor) or a false input that prevents the PCM from commanding them.
A couple things to check are below:
Cooling Fan Relays:
There are 2 or 3 relays (low/high fan) that can stick or burn out. Swap the relays with another identical one in the fuse box as a quick test.
PCM Not Commanding Fans:
If the coolant temp sensor is reading incorrectly (like stuck at 160°F), PCM won’t turn fans on. If the new A/C pressure sensor is giving a bad signal (wrong range or wiring issue), PCM may think pressure is low and not engage fans.
Since unplugging the A/C pressure sensor makes the fans run, check that the new sensor is correct for your year (different part numbers exist between years/engines). Check the 3-wire plug for bent pins, corrosion, or a shorted reference wire.
Start with those and let me know if you need additional assistance.
Sep 27, 2025 at 1:16 PM
