When the radiator fan refuses to turn on, the engine overheats at idle and in slow traffic - and cools fine at highway speed. Five to seven things cause this, ranked by frequency on modern cars.
The PCM uses the ECT sensor to decide when to turn the fan on. A failed sensor reads cold no matter what, and the fan never gets the command. Often paired with code P0117 or P0118.
Most cars have a high-current fuse and a relay for the fan circuit. Both are easy to swap and the cheapest place to start. Check under-hood fuse box first.
Brushes wear out, bearings seize, windings short. Jumper 12V directly to the motor - if it does not spin, the motor is dead.
Many late-model cars use a PWM module instead of a simple relay. Modules fail with heat and corrosion. Some cars (Ford, Chrysler) commonly need this.
Connectors on the fan motor sit in the worst location for heat and road grime. Pins corrode, wires chafe, grounds rust. Inspect at the motor connector.
AC normally triggers the fan. A stuck-open high-pressure switch leaves the fan off when AC is on. Test with a multimeter.
Rare, but a PCM output driver can fail and stop sending the fan signal. Last resort after ruling out everything else.
| Likely Cause | Typical Cost | DIY Difficulty | Severity | Likelihood |
|---|---|---|---|---|
| Bad Coolant Temp Sensor (ECT) | $15-$60 + 0.5 hr | Easy | High | 55% |
| Blown Fan Fuse or Bad Relay | $5-$25 | Easy | High | 45% |
| Failed Fan Motor | $80-$300 + 1-2 hrs | Moderate | High | 40% |
| Bad Fan Control Module | $150-$400 + 0.5-1 hr | Moderate | High | 30% |
| Wiring Harness Damage | $50-$200 + 1-2 hrs | Moderate | High | 25% |
| AC Pressure Switch Stuck | $25-$75 | Easy | Medium | 20% |
| PCM Output Driver Failed | $300-$1,200 | Pro Only | Medium | 15% |
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If your scanner is showing one of these, that is your starting point. Tap any code for full causes and repair costs.
Let the engine warm up to operating temp, turn the AC on full blast. The fan should kick on within a minute. If not, jumper 12V to the fan motor - spinning means motor is good, problem is upstream (relay, sensor, wiring).
Two different triggers. AC turns the fan on through the AC pressure switch. The hot-engine trigger goes through the ECT sensor and PCM. ECT sensor or PCM driver is the issue.
Only at sustained highway speed where ram air does the cooling. Stop-and-go traffic will overheat you within 5 minutes.
Most cars: $200-$400 installed at a shop. DIY part is $80-$200, labor 1-2 hours. Common job once you have the radiator shroud unbolted.
Indirectly. A thermostat stuck closed makes coolant in the engine hot but coolant at the sensor cold, so the PCM never sees high enough temp to trigger the fan. Fix the thermostat first.
Varies by car - usually a 40A or 50A in the under-hood fuse box, labeled "FAN" or "RAD FAN." Check your owner manual or the diagram inside the fuse box lid.
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