What is a Coolant Temperature Sensor?
A thermistor screwed into the cooling jacket that tells the ECU and gauge how hot your engine is.
Simplified animation — not to scale.
In plain English
A small two-wire sensor (a negative-temperature-coefficient thermistor) that sits in the coolant flow and changes its electrical resistance with temperature.
"Like the meat thermometer in your roast — the engine uses it to decide when it's done warming up and ready to run efficiently."
How it works
The ECU feeds a 5V reference through the sensor and measures the voltage drop across it. As coolant warms up the thermistor's resistance falls and the signal voltage climbs. The ECU uses this reading to enrich fuel on cold start, tweak ignition timing, trigger the radiator fan and drive the dashboard temperature gauge. A faulty sensor often reports "always cold", which makes the ECU dump extra fuel and ruins both economy and emissions.
Signs of trouble
- ⚠Black smoke and poor fuel economy
- ⚠Cold-start hesitation or stalling
- ⚠Cooling fan running constantly or never
- ⚠Temperature gauge reading wrong or stuck
£40–£120
Always get a written quote. Prices vary by car, region, and parts brand.