Coolant temperature sensor

When the engine is cold, additional fuel is required, and, just as the choke on a carburetor enriches the mixture by reducing the amount of air taken into the engine, the coolant temperature sensor signals the ECU to hold the injectors open longer when the engine is cold, passing more fuel into the engine.

The temperature sensor is inserted into the cooling system, much like the coolant temperature probe which activates the temperature guage or warning light. The coolant temperature sensor on Fiats is installed where the coolant enters the cylinder head. As the coolant warms the extra enrichment is reduced, until the engine is at normal operating temperature. The sensor is actually a thermo-resistor, where it's resistance decreases as it gets warmer.