Quick answer
A radiator fan that does not come on can cause overheating at idle or in traffic because there is little natural airflow through the radiator. At road speed, airflow may hide the fault temporarily.
The most common checks are coolant level, fan fuse, fan relay, wiring, fan motor, temperature sensor and whether the fan runs when the air conditioning is switched on.
Signs the radiator fan may not be working
Overheats in traffic
Temperature rises when stationary or moving slowly.
Idle overheating guide →Cools down when driving
Road airflow may cool the radiator once the car is moving.
Fan never comes on
The fan may not run even when the engine is hot.
Coolant warning light
Overheating or low coolant may trigger a dashboard warning.
Coolant warning guide →Air conditioning weak
Some cars need fan operation for correct A/C performance at idle.
Fan runs constantly
This can also suggest sensor, wiring or cooling system faults.
Common causes of a radiator fan not working
Blown fuse
A failed fan fuse can stop power reaching the fan circuit.
Faulty relay
A bad relay may stop the fan switching on when commanded.
Failed fan motor
The fan motor itself may fail, seize or become weak.
Temperature sensor fault
If the system does not see the correct temperature, it may not trigger the fan.
Wiring or connector issue
Corrosion, loose plugs or damaged wiring can interrupt fan operation.
Fan control module
Some cars use a controller or resistor pack to manage fan speed.
Low coolant level
Low coolant can cause overheating and may affect temperature readings.
Coolant loss guide →Airlock
Air in the cooling system can cause incorrect temperature behaviour.
ECU or sensor data issue
Modern fans may depend on several sensor inputs and control signals.
Does the fan come on with air conditioning?
On many cars, switching the air conditioning on will command the cooling fan to run or cycle, especially while stationary. If the fan runs with A/C but not when the engine gets hot, the fault may be linked to temperature sensing or fan control. If it never runs at all, the fuse, relay, wiring, module or fan motor may be suspect.
Fan works with A/C
Fan motor may be capable of running, so check temperature sensor, coolant level, fan command and control logic.
Fan does not work with A/C
Check fuse, relay, wiring, fan motor, resistor/module and A/C system condition.
Why a bad fan causes overheating at idle
When the car is moving, air naturally passes through the radiator. When the car is stationary, the radiator fan must pull air through the radiator. If the fan fails, engine temperature can rise quickly in traffic.
If the car overheats while driving at normal road speeds too, there may be another problem such as low coolant, thermostat restriction, blocked radiator, weak water pump or head gasket trouble.
Read more: engine overheating while driving.
Fan not working vs fan running constantly
Fan not working
Can cause overheating at idle, traffic temperature rises and poor A/C performance while stationary.
Fan always running
Can be caused by sensor faults, overheating, low coolant, A/C demand, wiring faults or emergency fan strategy.
What to check first
1. Watch the temperature gauge
If the temperature climbs, stop before the engine overheats badly.
2. Check coolant level when cold
Low coolant can cause overheating and should not be ignored.
3. Listen for the fan
The fan should usually run when the engine gets hot enough.
4. Try the A/C clue
On many cars, the fan should run or cycle when air conditioning is switched on.
5. Check fuse and relay
These are common failure points, but use the correct rating and location.
6. Book diagnosis
Fan systems can involve modules, sensors and ECU commands, so testing matters.
When to stop driving
- !The temperature gauge rises into the red.
- !The coolant warning light comes on.
- !Steam appears from the engine bay.
- !Coolant is leaking or the expansion tank is empty.
- !The heater blows cold while the engine is hot.
- !The engine runs rough, knocks or loses power.
Do not open the coolant cap while hot. Let the engine cool before checking coolant level.
Possible UK repair costs
Fuse or relay
Often lower cost if diagnosis confirms this is the only fault.
Temperature sensor
Usually moderate, depending on access and whether coolant needs draining.
Fan motor assembly
Can be more expensive if the fan, shroud or module comes as one unit.
Wiring repair
Cost depends on fault location, corrosion and diagnosis time.
Fan control module
Can vary widely by vehicle and system design.
Cooling system repair
If overheating is caused by low coolant or leaks, fan repair alone may not fix it.
Related coolant and overheating guides
Frequently asked questions
Why is my radiator fan not working?
Common causes include a blown fuse, faulty relay, failed fan motor, bad temperature sensor, wiring fault or fan control module problem.
Can a bad radiator fan cause overheating?
Yes. It commonly causes overheating when idling, sitting in traffic or moving slowly.
Is it safe to drive with the radiator fan not working?
It is risky, especially in traffic. Stop if the temperature rises, warning lights appear or steam is visible.
Should the fan come on with air conditioning?
On many cars, yes, but fan behaviour varies by vehicle and system design.
Can low coolant stop the fan working?
Low coolant may not directly stop the fan motor, but it can affect temperature readings and cause overheating symptoms.
Can a radiator fan run all the time?
Yes. Constant fan running can be caused by sensor faults, overheating, low coolant, A/C demand or emergency fan strategy.