What does it mean?
In this case the cylinder head temperature sensor circuit is electrically normal, but the value it reports does not fit the rest of the engine data. The module compares it with coolant temperature, intake air temperature, run time and engine load, and it expects the head temperature to sit close to the others after a soak and then to rise at a believable rate as the engine warms. This code is stored when the reading is stuck, drifted, rises too slowly or does not match the other temperatures. The fault can be the sensor itself, its thermal contact with the head, or a genuine cooling system problem that makes the head hotter or colder than the module expects.
Symptoms
- MIL on with head temperature that does not track coolant temperature during warm up
- Cooling fans switching on or off at the wrong time
- Temperature gauge and cabin heat that disagree with each other
- Higher fuel consumption and slightly rough running
- Reduced power on vehicles where a protection strategy responds to head temperature
- Overheating warnings that appear when the engine is not actually overheating, or none when it is
Likely causes
- Sensor drifted with age so it reads offset from the true metal temperature
- Poor thermal contact because of thread sealant, corrosion or debris in the sensor bore
- Sensor not tightened to specification, leaving an air gap between the tip and the head
- Low coolant level or restricted circulation so the head really is hotter than the coolant sensor suggests
- Thermostat stuck open or closed, giving a warm up pattern that fails the plausibility check
- High resistance in the signal or ground circuit shifting the value
- Incorrect replacement sensor with a different resistance curve
How to fix it
- After a full cold soak, compare head temperature with coolant and intake air temperature; all should be close before starting
- Verify with an infrared thermometer on the head near the sensor while the engine warms up
- Measure sensor resistance at a known temperature and compare it with the specification
- Remove the sensor and inspect the bore and threads for sealant, corrosion and debris, then refit to the correct torque
- Check coolant level, circulation and thermostat operation, since a real cooling fault will fail this test with a good sensor
- Test the signal and ground circuits for voltage drop and inspect the connector
- Replace the sensor with the correct part only once the cooling system and wiring are proven