Car systems
Engine cooling: how it works and what fails
The cooling system carries the engine's excess heat to the radiator with a liquid that flows through the block and keeps the engine at its working temperature, around 195 °F (90 °C). If the engine overheats, the head gasket and the engine itself can be damaged quickly, so the temperature warning light is a reason to stop.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
The water pump moves the coolant through the block and the cylinder head. While the engine is cold, the thermostat stays closed so it warms up sooner; once it reaches temperature, it opens and lets the coolant into the radiator, where the airflow from driving or the electric fan cools it.
The coolant temperature sensor reports to the engine computer, which uses it to adjust the mixture and the idle, switch on the fan and move the gauge on the dashboard. The expansion tank absorbs the coolant's expansion; the same circuit heats the cabin through the heater core, and the air conditioning adds its condenser in front of the radiator.
The most common faults
- Thermostat stuck open or closed: open, the engine is slow to warm up, uses more fuel and the heater blows lukewarm; closed, it overheats within minutes.
- Leaks and a low level: hoses, clamps, radiator, water pump or expansion tank; they leave colored residue and a sweet smell.
- Fan that does not start: relay, fuse, fan motor or its control; you notice it mostly in traffic, without the airflow from driving.
- Temperature sensor: a false reading can turn on the warning light, keep the fan running all the time or make cold starts hard.
- Head gasket: when coolant gets into the cylinders: lasting white smoke, a level that drops with no visible leak and bubbles in the tank.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Engine temperature warning light on: what to do
Stop as soon as it is safe
White smoke from the exhaust: what it means and what to do
See a garage soon
Low coolant warning light on: what to do
See a garage soon
Engine runs rough when cold and fine when warm: what it could be
Check without hurry
Car overheats in traffic and cools down when moving: what is wrong
See a garage soon
Engine cooling fan will not stop: when it is normal and when not
Check without hurry
Sweet antifreeze smell: what it means and what to do
See a garage soon
Car A/C not blowing cold: what it could be
Check without hurry
Car heater not blowing hot: what it could be
Check without hurry
The most relevant trouble codes
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
- P0217Engine coolant: over temperature
- P0115Engine coolant temperature sensor 1, circuit: malfunction
- P0116Engine coolant temperature sensor 1, circuit: range/performance
- P0117Engine coolant temperature sensor 1, circuit: signal low
- P0118Engine coolant temperature sensor 1, circuit: signal high
- P0125Insufficient coolant temperature for closed loop fuel control
- P0480Fan 1, control circuit: malfunction
- P0481Fan 2, control circuit: malfunction
- P0483Fan rationality check: malfunction
- P2181Cooling system: performance
- P0597Thermostat heater, control circuit open
- P2560Engine coolant level: too low
- P0526Fan speed sensor, circuit: malfunction
- P0530A/C refrigerant pressure sensor A, circuit: malfunction
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
- P01xxfuel and air metering
- P04xxauxiliary emission controls
- P00xxfuel and air metering and auxiliary emission controls
- P05xxvehicle speed, idle control and auxiliary inputs
- P06xxcomputer and output circuits
- P26xxoutput signals, intake heater and fuel pump
- P21xxthrottle, accelerator pedal and EGR
- P25xxturbo control
Questions and articles about this system
- What does the engine thermostat do and what does P0128 mean?
- What is live data and what is it for?
- What is freeze frame data?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
Presale price: €129 · US$129
Frequently asked questions
Can I keep driving with the engine overheating?
No. If the gauge reaches red or the light comes on, stop in a safe place and turn off the engine. Do not open the expansion tank while it is hot: the coolant is under pressure and can burn you.
Can I top up with water?
In an emergency, to reach a repair shop, yes; afterward the coolant the manual specifies has to go back in, because water alone does not protect against freezing or corrosion.
What does an OBD reader see of the cooling system?
The live coolant temperature and the codes of the thermostat (P0128), the sensor (P0115 to P0118) and the fan (P0480 and following). With the live temperature you can see whether the engine warms up and when the fan starts.