The coolant sensor is a thermistor (a resistor which changes value based on temperature) mounted on the engine coolant stream. As the temperature of the engine coolant changes, the resistance of the coolant sensor changes. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while high temperature causes low resistance (70 ohms at 130°C/266°F).
The ecm supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ecm determines the engine coolant temperature. The voltage will be high when the engine is cold and low when the engine is hot. This information is used to control fuel management, iac, spark timing, egr, canister purge and other engine operating conditions.
A failure in the coolant sensor circuit should either set a Code 14 or 15. These codes indicate a failure in the coolant temperature sensor circuit. Once the trouble code is set, the ecm will use a default valve for engine coolant temperature.