Radiator fan runs constantly
Cooling fan runs often or constantly; can be coolant temp sensor, AC request, or overheating.
MEDIUM RISK
Common symptoms
- Fan runs on cold start
- Fan runs after shutdown
- Temp gauge behaves oddly
Cost expectations
Typical costs vary by car and region. Use this as a ballpark.
- DIY / parts: £20–£80
- Garage / labour: £80–£250
Most likely causes (ranked)
Start at the top. Rule out the common, cheap causes before expensive ones.
- AC system requesting fan — Normal when AC is on.
- Coolant temp sensor fault — Reads hot; ECU commands fan.
- Thermostat issues — Temp swings; fan cycles oddly.
- Overheating due to low coolant — Fan running is symptom; find cause.
- Stuck fan relay — Electrical fault; fan stuck on.
Quick checks (fast, low-cost)
Fast checks you can do with basic tools before buying parts.
- Turn AC off and see if fan behaviour changes.
- Scan live coolant temp and compare to gauge/IR reading if possible.
- Check coolant level and leaks.
- Check for stored codes for ECT sensor.
- If fan runs with key off, suspect relay or ECU control issue.
Recommended tools
These tools make diagnosis faster and stop you buying random parts.
Related fixes
If your symptoms don’t match perfectly, check these next.
What to do next
- If coolant is low, fix leaks before further driving.
- If ECT reading is implausible, replace sensor and verify wiring.