Sensor malfunction diagnosis

A diagnostic system for detecting if a temperature sensor as found in motor vehicle climate control systems is faulty. The system can distinguish between a true sensor reading at extreme temperatures and a fault such as a short or open circuit. Firstly, the output from a sensor is checked to see whether it is beyond a certain high temperature (low voltage) limit (10). If so, then the sensor is deemed faulty (a short)(15). To distinguish between an open circuit and a reading at very cold temperatures, the system consults a second sensor exposed to the same temperature, to check whether it is in fact 'very cold' by comparing its reading with a particular chosen value (18). If this comparison reveals that the temperature is in fact very cold, then the output of the first sensor is deemed to be a true reading. In particular, combinations of ambient temperature sensors, in-car temperature sensors and evaporator air temperature sensors are tested by the system.