Car overheating in traffic or at idle: check the fan

What it means

The radiator is a network of thin channels that lets outside air cool the hot coolant before it returns to the engine. When the car is moving too slowly for airflow alone to do that, the cooling fan pulls the extra air through.

The fan motor is the part that does the work. A relay, or on many newer cars a fan control module, only switches it, and a faulty coolant temperature sensor can leave the fan running constantly or never. If the fan does not do its job, or debris is blocking the radiator, the engine loses exactly the airflow it depends on at idle and in traffic.

Common causes

• A failed cooling fan motor
• A fan relay or fan control module that no longer switches the fan
• A faulty coolant temperature sensor, which can leave the fan running constantly or never
• Blocked airflow through the radiator from debris, bugs or road dirt
• An actual cooling fault, such as low coolant from a leak, a thermostat stuck closed or a failing water pump

What to do now

1. Turn the air conditioning off. The compressor adds load and heat to the engine bay.
2. Turn the heater on full and roll the windows down.
3. Pull over as soon as it is safe and shut the engine off. If you see steam, do not open the hood.
4. Wait until the engine is fully cool, ideally at least 30 minutes, before checking the coolant level against the reservoir's min and max markings.
5. Look for a colored puddle under the front of the car.
6. Note what happened: whether the gauge climbed only in traffic or at idle, and whether the fan seemed to run constantly or not at all.

Fan, airflow, or the cooling system?

• The gauge climbs at idle or in slow traffic: often the cooling fan or its control, or blocked airflow through the radiator
• The fan runs constantly, or never: the coolant temperature sensor, the fan motor, the fan control module or relay, or an actual cooling fault
• Overheating with a coolant level that drops over time: a leak somewhere in the system
• Overheating although everything else looks normal: possibly a failing water pump
• Diesel, the fan running harder with a faint hot smell but no warning: often a particulate filter regeneration, which is normal
• White exhaust smoke together with overheating: deserves a same-day look

What people often get wrong

That overheating in traffic is a nuisance rather than a warning. Continuing to drive once the gauge climbs into the red is how a coolant problem becomes an engine problem, and damage from overheating often is not obvious until it has already happened.

When the car must go to a workshop

Diagnosing the cause of an overheating engine is a job for a professional, and so is any work that means opening the pressurized system. If the gauge reached the red, have the car inspected before driving it further.

Read next

• Coolant temperature light: the engine is overheating
• Car losing coolant: where is it going?
• Car heater blowing cold air: what it can mean

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.