Code 15, Coolant Temperature Sensor (Cts) Circuit Low Temperature Indicated

The coolant temperature sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on Light Green/Black wire to the sensor. When engine is cold, thermistor resistance is high. This means the ECM will see high signal voltage. As engine warms, thermistor resistance is less. This means the ECM will see low signal voltage. When engine reaches normal operating temperature, signal voltage will be approximately 2.5-3.0 volts at ECM terminal No. 17. Coolant temperature is an input used by the ECM to control "Open Loop" timing, fuel delivery and early fuel evaporation.

1. This checks if code was set as the result of a hard failure or an intermittent condition. Code 15 will set if signal voltage indicates a coolant temperature less than 32°F (0°C) after engine has been running for 5 minutes.
2. This simulates conditions for Code 14. If ECM sees grounded circuit (low voltage) and displays high coolant temperature, the ECM and wiring are okay.
3. This determines if wiring or ECM is faulty. If circuit No. 452 is open, Code 33 may also be stored. Ensure engine harness connectors are clean and tight.