You are stopped at a light in the city centre, 32 degrees outside, the temperature needle is slowly creeping up, and under the hood there is silence. None of that familiar humming from the radiator fan. The opposite scenario: you switch the engine off, step out of the car, and something under the hood is still running at full speed. Both situations point to the same component, but only one of them is actually a problem.
What the radiator fan does and when it kicks in
The radiator fan is there to push air through the radiator when natural airflow can no longer cool it down, which in practice means crawling in traffic, sitting at a light, and idling. While you are driving on the open road, air comes in through the front grille on its own and cools the coolant; the fan often does not even need to switch on.
It turns on in two ways. The first is via a thermal switch on the radiator or a signal from the ECU (engine control unit), once coolant temperature crosses a set threshold. Those thresholds vary from car to car, but as a rough guide they sit around 95 to 105 degrees for the first speed and a bit higher for the second, stronger speed. The second way is through the AC: as soon as you switch the air conditioning on, the fan usually starts up regardless of engine temperature, because the AC condenser sitting in front of the radiator also needs cooling.
Symptoms a driver actually notices
The classic sign is the car overheating in city traffic while temperature stays perfectly normal on the open road. That is almost a signature for a fan fault, because while you are driving air passes through the radiator without the fan's help.
The second symptom is weak AC cooling at standstill, while it blows ice cold once you start moving. If the AC condenser has nothing cooling it, system pressure rises and the AC throttles itself back. The third symptom is the opposite: the fan runs constantly, the moment you turn the key, regardless of temperature. That usually means the thermal switch is shorted or the temperature sensor is lying to the ECU, so the computer holds the fan at maximum just to be safe.
The fan running for 5 to 10 minutes after you shut the engine off is not, by itself, a fault. That is the cylinder head and turbo cooling down, especially on turbocharged engines after a longer drive, and it is built into the design. You only need to worry if it never shuts off, or if it runs for 20 minutes or more on a cold engine.
The most common causes
In the vast majority of cases the culprit is one of these five. First, a burnt-out fan motor, usually because of worn brushes or shot bearings. Second, a blown relay or fuse in the fuse box; the relay is a cheap part that does its work and tends to fail first. Third, a faulty thermal switch on the radiator that simply will not close the circuit when it should. Fourth, a bad coolant temperature sensor, so the ECU does not know the real temperature and never sends the command to switch the fan on. Fifth, damaged wiring or a corroded connector, most often right at the fan motor itself where it is exposed to moisture and road salt.
On some older cars and certain longitudinally-mounted-engine diesels, instead of an electric fan there is a mechanical viscous fan driven directly off the engine pulley. Diagnosis there is completely different, because what fails is the viscous clutch, not a motor, and you can spot it by the fan turning weakly when the engine is hot.
Quick at-home checks before the workshop
Let the engine idle until it reaches operating temperature, then let it climb a touch above. Switch the AC on full. The fan should start up, usually within a minute or two. If you hear nothing, the first thing to check is the fan fuse; the label is on the inside of the fuse box cover.
If the fuse is good, while the signal is active, have someone gently tap the fan motor housing with a wooden handle. If the fan kicks in right after the tap, that is a sign of worn brushes and the motor needs replacing. The other mechanical clue is the fan blades being hard to turn by hand when the engine is fully cold and off, with squeaking or binding, which points to a failed bearing.
Bypassing the fuse and wiring the motor straight to positive is not a solution, not even as a temporary fix for any longer period. You can do it as a brief field test, only to see whether the motor spins at all, and never leave it like that, because you lose thermal protection and can burn the fan out completely.
When you must stop, and what it means when the fan runs after shutdown
If the temperature needle climbs past the normal zone and starts heading for the red, especially if you see steam from under the hood or smell that sweet coolant smell, pull over as soon as it is safe and shut the engine off. Open the hood so air can circulate and wait 20 to 30 minutes for everything to cool before you check coolant level. Driving on with an overheated engine is the shortest route to a blown head gasket and a warped cylinder head, and that repair can run higher than the value of the car itself.
As for the fan continuing to run after you shut the engine off, in 90 percent of cases that is normal system behaviour, not a fault. The ECU stays awake for a few more minutes and, through a separate relay, keeps the fan running until temperature drops below a safe threshold. You only need to worry if it goes on too long, drains the battery quickly, or runs even when the engine is fully cold.
If you are noticing any of the above, especially overheating in town, stop by the workshop for a check. The gap between swapping a relay or a motor and rebuilding an engine because of a blown gasket is far too wide to leave to chance. Better to check now than regret it later.