Car overheating but coolant is full: five causes to check
Updated 23 July 2026 · OBDCode UK editorial
Can you keep driving?
Do not continue driving an overheating engine and never open a hot pressure cap. Pull over safely, switch off and recover the car if temperature rises again. Cabin heat on is an emergency measure to reach a safe stopping point, not permission to finish the journey.
Match what the car is doing
| What you notice | Most likely | How to separate it |
|---|---|---|
| Overheats in traffic, cools when moving | Cooling fan, relay or airflow problem | Command the fan with diagnostics and check power/ground; do not put hands near it. |
| Overheats at speed as well | Thermostat, water pump, blockage or head gasket | Temperature drop across radiator, pressure test and gas test narrow it down. |
| Gauge hot but hoses/coolant stay cool | Sensor, wiring or thermostat not opening | Compare scan temperature with an independent measurement. |
| No cabin heat while gauge rises | Air lock or poor coolant circulation | Stop immediately; bleeding procedure and pump flow need checking. |
Checks to do before buying parts
- Wait until stone cold, then verify the level and look for staining, crust or dampness.
- Read coolant-temperature data and fan-related codes such as P0480.
- Pressure-test the system; small hot leaks often leave no puddle.
- Use a combustion-gas test only as one clue, then confirm before authorising head work.
Typical UK costs
| Likely job | Typical fitted price |
|---|---|
| Thermostat/housing | £120–£500 |
| Cooling fan/relay/wiring | £80–£600 |
| Water pump | £220–£750 |
| Radiator | £250–£500 |
| Head-gasket repair | £700–£1,700+ |
Indicative 2026 independent-garage prices for mainstream cars; London, prestige, SUV and seized-part jobs can cost more. Diagnose first.
MOT relevance
The MOT is not a full cooling-system health check, but a visible serious leak, warning lamp where applicable, or unsafe condition can fail. Overheating damage is far costlier than postponing a test.
- Power Probe circuit testersTrace live circuits and voltage drops safely
Frequently asked questions
Can a car overheat with no coolant leak?
Yes. Coolant may be full but not circulating because of a thermostat, pump, air lock or blockage; combustion gases can also pressurise the system.
Why does it overheat only when idling?
Road speed is no longer pushing air through the radiator, so the electric fan system becomes the first suspect.
Sources and further reading
General guidance for UK drivers, not professional advice. MOT rules follow the DVSA inspection manual (2026 edition).