The engine coolant temperature (ECT) sensor tells the engine computer how hot the engine is. The computer uses it for warm-up fueling, ignition timing, idle speed, cooling fan control and transmission shift timing, and on many vehicles it also drives the temperature gauge. A bad coolant temperature sensor can cause poor fuel economy, black smoke, hard starting, fans that run all the time and a gauge that reads wrong.
Bad coolant temperature sensor symptoms
- Poor fuel economy and black smoke, because the computer thinks the engine is cold and adds fuel
- Hard starting, especially when the engine is warm
- Cooling fans running constantly, or not running when they should
- Temperature gauge reading cold, pegged hot or erratic
- Rough idle and harsh shifting on some vehicles
Coolant temperature sensor codes
| Code | Meaning |
|---|---|
| P0117 | Circuit low (reads very hot or shorted) |
| P0118 | Circuit high (reads very cold or open) |
| P0128 | Engine not reaching operating temperature (often the thermostat) |
| P0217 | Engine over temperature |
How a coolant temperature sensor works
Most ECT sensors are thermistors: their resistance is high when cold and drops as the engine warms. The computer supplies a 5-volt reference and reads the voltage, so a cold engine gives a high voltage and a hot engine a low one. An open circuit reads as extremely cold (often about -40 degrees on a scan tool) and a short reads as extremely hot.
How to test it
- On a cold engine that has sat overnight, the coolant reading should match outside air temperature within a few degrees.
- Warm the engine and compare the scan tool reading with an infrared thermometer pointed at the thermostat housing.
- Check resistance at different temperatures against the chart in your service information.
- Inspect the connector for corrosion and coolant contamination.
Sensor, gauge sender or thermostat?
Many engines have a separate gauge sender, so the dash gauge can read correctly while the computer sensor is wrong, or the reverse. A gauge that stays low on the highway and slow heater warm-up usually means a thermostat stuck open rather than a sensor; see water pump and thermostat symptoms.
Replacing the sensor
Replace it with the engine cold. Expect to lose a little coolant; top it up and bleed the cooling system afterward, because an air pocket at the sensor tip gives false readings. If the engine really has been overheating, check for a coolant leak or a blown head gasket; a badly overheated engine may need replacing, and our engines page lists options by code.
More guides: heater blowing cold air · bad thermostat symptoms.
Trouble codes: P0115 Coolant Temp Sensor Circuit · P0116 Coolant Temp Range/Performance · P0125 Insufficient Coolant Temp for Closed Loop · P0119 Coolant Temp Sensor Erratic · P0126 Insufficient Coolant Temperature · all OBD2 codes.
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