Engine Temperature Sensor: Description and Operation

The CHT sensor:

^ is mounted to the wall of the cylinder head and is not connected to any coolant passages.
^ sends the PCM a signal indicating CHT.

^ As the CHT rises above normal operating temperature, the PCM will turn on the high speed cooling fan. If the temperature continues to rise, the PCM will selectively and alternately disable fuel injectors. The cylinders that are not being fuel injected will act as air pumps to aid in engine cooling. The PCM will also turn on the check engine light and set a DTC. If the CHT continues to rise, the check engine light will begin to flash and an additional DTC will be set. Finally, if the CHT rises still more, the PCM disables all of the fuel injectors, turning the engine off.

Typical CHT Sensor:

The CHT sensor is a thermistor device in which resistance changes with the temperature. The electrical resistance of a thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.

The CHT sensor is installed in the cylinder head and measures the metal temperature. The CHT sensor can provide complete engine temperature information and can be used to infer coolant temperature. If the CHT sensor conveys an overheating condition to the PCM, the PCM initiates a fail-safe cooling strategy based on information from the CHT sensor. A cooling system concern such as low coolant or coolant loss could cause an overheating condition. As a result, damage to major engine components could occur. Using both the CHT sensor and fail-safe cooling strategy, the PCM prevents damage by allowing air cooling of the engine and limp home capability.