What does it mean?
The cylinder head temperature sensor is a thermistor: its resistance falls as the metal gets hotter, so a hot head produces a low signal voltage at the module input. This code is stored when the signal drops below the lowest voltage the sensor can realistically produce and stays there, which the module reads as a temperature far beyond anything possible. The usual cause is the signal wire touching ground or a shorted sensor element, not an overheating engine. Because the module now believes the head is extremely hot, it may run the fans continuously and cut power to protect the engine.
Symptoms
- Displayed engine or head temperature extremely high with a cold engine
- Cooling fans running at full speed from the moment the engine starts
- MIL on, often with a false overheating warning
- Reduced power or limp mode as the module protects an engine it believes is overheating
- Lean mixture and hard cold starting, because the module fuels as if the engine were fully warm
- Poor idle quality when cold
Likely causes
- Signal wire shorted to ground through chafed or heat damaged insulation near the exhaust
- Internally shorted sensor element
- Coolant, water or corrosion in the connector bridging the signal and ground terminals
- Terminals bent or pushed together inside the connector housing
- Harness pinched under a bracket or crushed during previous engine work
- Incorrect sensor with a much lower resistance curve fitted at an earlier repair
- Rarely, a shorted input inside the control module
How to fix it
- Compare the live head temperature with an infrared reading of the actual metal before assuming an overheat
- Unplug the sensor with the key on; if the reading swings to the extreme cold end the harness and module are fine and the sensor is shorted
- If the value stays at the hot limit with the sensor unplugged, the signal wire is shorted to ground
- With the module connector disconnected, measure resistance from the signal wire to ground to locate the short
- Inspect the connector for coolant, water and corrosion, and check for terminals pushed into contact
- Repair the damaged harness section and protect it from exhaust heat
- Replace the sensor when the wiring proves good, using the correct part number for that engine