Code 15, Coolant Temp. Sensor Circuit High Temp. Indicated

The coolant temperature sensor uses a thermistor to control signal voltage to ECM. ECM applies voltage on the Gray/White wire to sensor. When engine is cold, thermistor resistance is high. ECM will sense high signal voltage. As engine warms, thermistor resistance is less. ECM will sense low signal voltage. When engine reaches normal operating temperature of approximately 176-203°F (80-95°C), signal voltage will be approximately 1.5-2.0 volts at ECM. Coolant temperature is one of the inputs used by ECM to determine fuel delivery and ignition timing.

1. Code 15 will set if signal voltage indicates coolant temperature is more than 276°F (135°C) for 3 seconds.
2. Checks Gray/White wire for a short to ground, creating conditions for Code 15.