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.