What does it mean?
Some engines carry a cylinder head temperature sensor screwed directly into the metal of the head so that the module can see the temperature of the casting rather than that of the coolant. That reading supports fuelling, spark protection and, on several designs, a fail safe cooling strategy that allows the engine to be driven a short distance after coolant loss. Sensor A is the primary unit on engines fitted with more than one. This code is the general circuit fault: the module cannot obtain a valid signal from the sensor, and the problem lies in the sensor, its wiring or its connector rather than in an actual temperature condition.
Symptoms
- MIL (check engine lamp) on, often with a temperature warning or a strange gauge reading
- Cooling fans running constantly or not running at all
- Rich running, black smoke or hard starting because the module uses a default temperature
- Reduced power on vehicles that apply an engine protection strategy
- Poor fuel economy and rough running while warming up
- Cabin heat that does not match what the instrument display shows
Likely causes
- Open or short in the signal or ground wire between the sensor and the module
- Connector corroded, heat damaged or not fully seated at the sensor
- Internally failed sensor element
- Harness melted or chafed where it passes close to the exhaust manifold
- High resistance sensor ground or a corroded ground splice
- Sensor loose in its thread so it does not make proper contact with the head
- Rarely, a damaged input pin or driver inside the control module
How to fix it
- Read the live value and compare it with coolant temperature after a long cold soak; before starting all temperatures should be similar
- Inspect the connector for heat damage, corrosion, melted insulation and backed out terminals
- Check for the reference voltage at the sensor connector with the key on
- Jump the signal terminal to ground at the connector and watch whether the displayed temperature swings to its opposite limit, which proves the harness and module input
- Measure sensor resistance and compare it with the temperature specification
- Confirm the sensor is correctly torqued in the head with clean threads and good metal contact
- Repair the harness or connector where the fault is found, and replace the sensor only when the circuits test good