What does it mean?
Because the cylinder head temperature sensor is a thermistor, a cold head means high resistance and therefore a high signal voltage. This code is stored when the signal stays above the highest voltage the sensor can realistically produce, which the module interprets as an impossibly cold head. An open signal wire, an open sensor ground or a sensor that has failed open all leave the module input floating at reference through its internal pull-up, so the symptom is the same in each case. The module then fuels as though the engine were extremely cold, which makes the mixture very rich.
Symptoms
- Displayed head temperature stuck at the extreme cold end of the scale even with a hot engine
- Very rich running, black smoke and a strong fuel smell
- Poor fuel economy and fouled spark plugs
- Difficult hot starting and rough idle once the engine is warm
- Cooling fans that do not switch on when they should
- MIL on, sometimes with fuel trim or catalyst codes stored alongside
Likely causes
- Open circuit in the signal wire between the sensor and the module
- Open or high resistance sensor ground circuit
- Internally open sensor element
- Connector unplugged, not locked home, or with a terminal that has backed out
- Corroded or heat damaged terminals producing a resistance high enough to act as an open
- Signal wire shorted to a voltage feed running in the same harness
- Rarely, an internal module fault holding the input at reference
How to fix it
- Read the live value with a hot engine and confirm it is at the cold limit rather than genuinely wrong by a few degrees
- Jump the signal terminal to ground at the sensor connector; if the display swings to the hot limit, the wiring and module input are good and the sensor is open
- Measure the sensor ground circuit for continuity and voltage drop back to its ground point
- Check signal wire continuity from the sensor to the module while flexing the harness
- With the sensor unplugged, test for voltage on the signal wire to rule out a short to a supply
- Inspect the connector for heat damage, corrosion and terminals that have lost their grip
- Replace the sensor once the circuits prove good, then verify the reading matches an infrared measurement of the head