Code 15, Coolant Temp Sensor (Cts) CKT, High Temp Indicated: Notes

The coolant temperature sensor is a thermistor in series with a fixed resistor in the ECM. The ECM applies 5 volts to sensor. ECM monitors voltage across the CTS and converts it into a temperature reading. When engine is cold, CTS resistance is high. When engine is warm, CTS resistance is low. Therefore, when engine is cold, ECM receives a high voltage input and when engine is warm, ECM receives a low voltage input.

Code 15 will set if the following condition is met for at least 3 seconds:

Voltage input at ECM indicates a coolant temperature of greater than 276°F (136°C).
NOTE: Test number refers to number on diagnostic chart.

This test simulates Code 14. If ECM sets Code 14, ECM and its wiring are okay and CTS is faulty. If Code 15 resets, CTS is okay and the wiring to ECM or the ECM is faulty.