Code 15, Coolant Temperature Sensor (Temperature Too Low)

The coolant temperature sensor uses a thermistor to control signal voltage to ECM. The ECM supplies voltage on circuit No. 410 to sensor. When engine is cold, the sensor resistance is high, and ECM receives a high voltage signal. As engine coolant warms, sensor resistance becomes less, and voltage signal drops. At normal engine operating temperature, voltage will measure about 1.5-2.0 volts at ECM terminal C10.

Coolant temperature is one of the inputs used to control:

Fuel delivery.
Electronic Spark Timing (EST).
Radiator cooling fan.
Torque Converter Clutch (TCC).
Idle Air Control (IAC).

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

Check if code will set as a result of hard failure or intermittent condition. Code 15 will set if:
- Engine has been running for more than one minute.
- Signal voltage indicates coolant temperature below -35°F (-37°C).
This tests simulates conditions for Code 14. If ECM recognizes a grounded circuit (low voltage), and displays a high temperature, ECM and wiring are okay.
This test will determine if there is a wiring problem or defective ECM. If circuit No. 452 is open, there may also be a Code 33 stored.