Code 15, Coolant Temp Sensor CKT/High Temp Indicated

The Coolant Temperature Sensor (CTS) uses a thermistor to control signal voltage to ECM. ECM applies voltage on Red/Yellow wire to coolant sensor. When engine is cold, sensor resistance is high. ECM will sense high signal voltage. As engine warms, sensor resistance lowers and voltage drops. With engine at normal operating temperature, 176-203°F (80-95°C), voltage will be about 1.5-2.0 volts at ECM.

NOTE: Test numbers refer to test numbers on diagnostic chart.

If signal voltage indicates coolant temperature is greater than 285°F (140°C), Code 15 will set.
Checks if Red/Yellow wire is shorted to ground, creating conditions for Code 15.