Coolant Temperature Sensor
What it is
The engine coolant temperature (ECT) sensor is a small sensor that reads the temperature of the engine coolant and reports it to the computer (and often the dashboard gauge).
What it does
The computer uses coolant temperature to set the cold-start mixture, fan operation, and when to switch into closed-loop running. A faulty sensor can cause hard cold starts, poor economy, a wrong temperature gauge, and codes P0116–P0118 or a P0128.
Symptoms of failure
- Codes P0116, P0117 or P0118 (or a contributing P0125/P0128)
- Hard starting when cold
- Poor fuel economy / black smoke
- Cooling fan running constantly or not at all
- Inaccurate temperature gauge
Common fault codes
Which vehicles need it
Every liquid-cooled engine. Some engines have two sensors (one for the computer, one for the gauge).
Replacement cost
| DIY (part only) | $15–$50 |
|---|---|
| At a shop (parts + labor) | $110–$250 |
| Replacement interval | No set interval — replace on failure. |
| DIY difficulty | Easy–Moderate (DIY) — one connector; you may lose a little coolant |
| Recommended brands | Standard Motor Products, Bosch, Delphi, ACDelco |
Where to buy the part
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Frequently asked questions
What is the difference between the ECT sensor and the thermostat?
The thermostat is a mechanical valve that controls coolant flow; the coolant temperature sensor is an electronic probe that reports temperature to the computer. Both can cause a P0128, so check which one is at fault.
Can a coolant temperature sensor cause poor fuel economy?
Yes. If it wrongly reports a cold engine, the computer keeps adding extra fuel, hurting economy and sometimes causing black smoke and rough running.