An exhaust gas temperature sensor measures the temperature of the exhaust gas. This information is then relayed back to the engine control unit or ECU, where the 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.
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 not uncommon 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
There are two types of exhaust gas temperature sensors; one with a positive temperature coefficient (PTC) sensing element, and the other negative temperature coefficient (NTC), the only difference being how they measure the temperature. The NTC element has a high resistance at low temperatures and a low resistance at high temperatures. Resistance decreases as the the temperature increases. The PTC element, the most common type resistance increases in line with temperature. Either way a temperature is assigned to the resistance in the ECU and action is taken accordingly.
Common causes of exhaust gas temperature sensor failure is exposure to excessively high temperatures - over 900°C in some cases, the very thing they protect other components from.
As with all wired sensors, severe vibrations can also loosen internal connections and any bends or twists can cause the wire to break, making them particularly vulnerable to damage when replacing other components in the exhaust system.
Alongside contamination from other fluids such as oil or antifreeze, these can all affect the sensor's response characteristics, causing it to drift out of tolerance and prove inaccurate readings.
Since the oxygen sensor is in the exhaust stream, it can become contaminated. Common sources of contamination include an excessively rich fuel mixture condition or oil blow-by in an older engine and engine coolant being burnt in the combustion chamber as a result of on engine gasket leak.
It is also exposed to extremely high temperatures, and like any component can wear over time. These can all affect the oxygen sensor's response characteristics, resulting in an extended response time or a shift in the sensor voltage curve and in the long run reduced sensor performance.