Radiator fan runs when car is cold but not hot

Try replacing the cooling fan relay as a first step.

The A/C causes the engine to work harder (that's why fuel economy suffers when it is run).

If your car has an electric fan - make sure it runs when the engine is hot. If it does not - you may have trouble w/o the A/C running if is gets warmer outside. Look for fail fuses, disconnected plugs, etc.

You may also have flaky thermostat that is not opening fully when needed to exchange hot water in the engine block with cooler water in the radiator.

First, check the coolant level. You should find a semi-transparent reservoir / tank located under the hood that has a cap marked "Coolant". The side of the tank should have two indicator lines - one for HOT and one for COLD. If the engine has been run up to normal temperature (so that it can provide heat in the car), you should see the coolant level at or slightly above the HOT line. If the engine is cold - first thing in the morning or after sitting for several hours - the coolant should be at or slightly above the COLD line. View these levels while the engine is off and the car is parked on a flat, level surface. If there is not enough coolant - you will need to add the CORRECT type for your car.

Hot Engine
(1) With the engine hot, add enough coolant to the tank to bring the level to the HOT line. You may have to check the tank several times after driving if the coolant level in the radiator is down a great deal. NEVER open the radiator cap when the engine is warm or hot!

or:
Cold Engine
(2) With the engine cold, add enough coolant to the tank to bring the level to reach COLD line. Additionally - with the engine cold - you can add coolant directly to the radiator by removing the cap. Start the engine, set heat in car to HOT and add coolant mixture to radiator until level comes up to the bottom of the filler neck. Once the engine is HOT (getting heat in the car) check and top off radiator as needed and put cap back on radiator.

Check the levels in the TANK several times over the next few days when cold. If the tank ever empties -you'll need to have this looked at by a mechanic.

There are certain positions for the climate control system that will cause the A/C system to activate. When the A/C is on, the radiator fans automatically run to provide airflow thru the condenser coils in front of the radiator. This could be the issue. Turn off the climate controls and recheck.

Hi Joe. For your information you never but never fill a radiator when the engine is hot and engine running. Must first have a cool engine open the radiator and if is only topping up coolant be first sure that the water is clean and fresh ( drain the old coolant first ) and after leaving a space for the coolant top up and always not overfill . Then you start the engine and let it run in idle with radiator cap off, turn also the heater on to hot air for helping the coolant to circulate. When you see that the temperature rise close the radiator and continue with engine on idle until the fan starts. Leave the car running till the fan cut out. Switch engine off,let it cool down - for dropping the pressure - and after you do open the radiator to check for any need for topping up coolant. As soon as you have completed just close the radiator.

Possibly leaking head gasket. Easiest way to check would be to fill radiator full, start engine and watch coolant level in the radiator cap opening. The top radiator hose will become hot when thermostat begins to allow coolant flow through the radiator. You may have to top off the coolant. Still with the radiator cap off watch for water belching out of the radiator neck. If coolant does belch out of the radiator cap neck, refill with water. If it belches out coolant again it would be combustion gassed forcing the coolant out of the radiator.


This process should take about 1/2 hour to diagnose if it is a head gasket leaking combustion gasses into the coolant.


The radiator fan may run once car is shut off to cool engine compartment to prevent gasoline in fuel lines from vaporizing which would cause a condition called "vapor lock". A car won't start if fuel lines are vapor locked.

Good luck, I hope this helps you to figure out the problem.

I would recommend first to check the coolant level. If the coolant level is too low it can cause the fan to run if it detects too low of a coolant level. Also, the low warning can be a result of a low level. Make sure to check the level with the engine cold, and not running.

Assuming the check engine light is not on, I would want to know what the engine temp sensor is doing.
A scanner would tell you what the temp sensor is showing - if it is unplugged or there is a problem with the wiring the fans will run as a default to prevent the engine from overheating. It could be a fan relay but I doubt it. A relay usually works or not.

The fans are radiator cooling fans. They are temperature controled so when you first start your engine in the morning they should not run - UNLESS you start it with your AC on. If you turn your AC on one of them should start operating and the other one will kick on when your engine starts getting hot.

The cooling fan is made to come on when the engine reaches a certain temperature. To give a simple and general test - turn the AC on high - it your AC and fan are both working correctly the fan should come on almost immediately. If not within very few minutes like 5 or 8.
Also at operating temperature the fan will run as well. In the early morning before you start the car open the hood - fill the radiator while the engine is still cold. start the engine and wait about 10 minutes or so. The fan should come on before the engine over heats. This may take up to 20 minutes and the fan is designed to run after the car is shut off IF - the temperature is hot enough.

The fan is only supposed to come on when there is excess heat .. in normal driving conditions it doesn't start. When you start your car in the morning the fan won't come on because the engine is cold.

With a hot engine - and it depends how hot the engine is - the fan will only kick in to prevent overheating. In heavy slow moving traffic or a traffic jam, the fan will kick in if needed.

The air flow through the radiator is usually sufficient -in normal driving conditions - to cool the engine.

When you park up and the car has a hot engine, the fan will kick in. This is perfectly normal. It is caused by a surge of hot coolant triggering the fan sensor. Perfectly normal.

The fan then should cut out after a while .. maybe 2-3 minutes at the most. If the fan runs continuously that suggests a fault in the temperature sensor switch.

Next time - wait by the car and see how long it takes for the fan to quit after you have parked up. It should stop of its own accord after a couple of minutes.

If the fan cuts off after a short while, and your battery is becoming drained, that's probably a battery or alternator fault.

Remember: when you start the car the fan won't kick in at idle speed because the engine is COLD, and there is no reason for the fan to start up ..
The fan WILL kick in for a couple of minutes after you have parked the vehicle and the engine is hot