2018 Ford F150 3.5 EcoBoost Radiator Fan Runs Constantly After Repair

Most likely, the PCM thinks the engine is running hot due to a bad coolant temp sensor or a wiring issue. Since the fan stayed off after the water pump swap, but the problem returned, I’d bet on a sensor or harness problem, not a stuck relay or module. If the temp sensor reads high, the PCM will command the fan on, even in winter. Second runner-up: the fan control module is sticking or getting a bogus signal.

Most likely culprits

• Faulty engine coolant temperature (ECT) sensor or wiring — If the sensor reads too hot, the PCM will run the fan.
• Fan control module acting up — If it’s sticking or misreading signals, it’ll keep the fan on.
• A/C pressure sensor issue — If the PCM thinks A/C pressure is high, it’ll run the fan, even with A/C off.

How to narrow it down

1. Scan for codes and live data — Check for pending/history codes in PCM and BCM. Look at ECT and A/C pressure readings with engine cold and at temp. If ECT is high with a cold engine, suspect the sensor or wiring.
2. Check ECT reading on cold start — Before starting, ECT should match ambient temp. If it’s way off, the sensor or wiring is bad.
3. Watch fan operation with A/C off — Turn off A/C, see if fan still kicks on. If yes, A/C pressure sensor is less likely.
4. Wiggle test harness at ECT sensor — If fan cycles or ECT reading jumps, you’ve got a wiring issue.
5. Check for TSBs or PCM updates — Ford’s had logic updates for fan control on some models.

Possible parts (if needed)

• Engine coolant temperature sensor
• Radiator fan control module
• A/C pressure sensor (less likely)

A few questions

• Does the temp gauge on the dash read normal, high, or low when the fan is running?
• Is the A/C or defrost on when the fan kicks in, or does it happen with HVAC off?
• Any recent wiring work, rodent damage, or aftermarket installs near the engine harness?