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

The coolant temperature sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage on the Gray/White 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 approximately 176-203°F (80-95°C), signal voltage will be approximately 1.5-2.0 volts at the ECM. Coolant temperature is one of the inputs the ECM uses in determining 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. This checks Gray/White wire for a short to ground causing the conditions for a Code 15.