Exhaust gas temperature sensors use resistors whose resistance changes with temperature. There are two types of sensors in wide use: those using positive temperature coefficient (PTC) thermistors and those using negative temperature coefficient (NTC) thermistors. The exhaust gas temperature sensors are equipped with heat-resistant sensors operating in the range from -40 ° C to +900 ° C.

The temperature information is relayed back to the engine control unit (ECU), where appropriate action is taken. In petrol engines, its main role is to protect key components from the higher temperatures common with downsized engines, so if the sensor detects excessive temperatures, the ECU will reduce the temperature by lowering boost pressure in the case of the turbocharger, for example, or increasing the quantity of fuel injected for the catalytic converter. In diesel engines, exhaust gas temperature sensors are also used to monitor the temperature of the diesel particulate filter (DPF) to establish the correct temperature for regeneration, reducing harmful emissions. It is common to have three or more sensors fitted to the exhaust; one prior to the turbocharger, one before the DPF and the third after the particulate filter.

Primary failure modes

Burn-out from high temperature, circuit damage and signal misalignment. These, alongside contamination from other fluids such as oil or antifreeze, can all affect the sensor’s response characteristics, causing it to drift out of tolerance and provide inaccurate readings.

The main causes of the failure are poor temperature resistance of materials, poor strength and durability, circuit damage after high and low temperature and vibration. Signal misalignment is mainly caused by poor welding quality and chip quality.

By Andy RS ZHANG

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