The cooling fan will not run: where to look

The fan matters where there is no airflow: in traffic, at lights, when parked. So its failure is usually discovered in the worst possible place — a summer jam.

A two-minute test

Switch on the air conditioning with the car stationary and the engine running. On the vast majority of cars the fan should start almost immediately, regardless of engine temperature.

It started — the fan and its power circuit are alive. So the problem is temperature-based control: the sensor, the module, or another relay.

It did not start — look at the supply: fuse, relay, motor.

That test settles half the cases with no instruments at all.

The usual causes

Relay and fuse. The first thing to check: swap the relay with an identical one, test the fuse; covered in the fuses and relays article.

The temperature sensor. If it under-reads, the module concludes it is too early to run the fan. The engine heats up and the fan stays silent — and the dashboard needle may lie too, because it runs from the same signal.

A resistor or fan speed module. The typical symptom: it runs only on high speed, or only with the air conditioning.

The motor itself. Worn brushes or a seized bearing. The sign: it starts after a tap or runs intermittently.

Wiring — typically in the engine bay, where heat and vibration live.

Symptoms that point at the fan

Temperature rises in traffic and falls on the move. That is the fan’s signature: at speed the airflow does its job for it.

Air conditioning cools on the move but not in traffic — the same fan draws air through the condenser.

The fan runs constantly from start-up — also a fault: usually the sensor or a stuck relay.

Why not to run it on

Overheating in traffic is not “stop, cool down, carry on”. The damage accumulates: a warped head, a blown gasket, distorted pistons; the general case is covered in the overheating article.

A roadside tip: if the temperature climbs in a jam, put the heater on full with the blower at maximum. The heater matrix acts as an extra radiator and buys time to get off the road.

What diagnostics shows

Coolant temperature in live data — the real one, not the needle, which on many cars is damped and holds “normal” until the last moment.

Fan request status — whether the module is asking for the fan. This is the key parameter: request present but no fan means the power side; no request means the sensor or the logic.

Fan and temperature sensor codes — P0480, P0117, P0118.

Air conditioning status — to correlate with the test above.

In short

• Switching on the air conditioning is a two-minute fan test.
• Hot in traffic, cool on the move is the fan’s signature.
• A sensor that under-reads gives a silent fan and a lying needle.
• Module request versus actual operation separates control from power.
• The heater on full buys time to get out of a jam.

Download the app — temperature, fan request, and codes are read with an ELM327 adapter over Bluetooth SPP on Android.