The coolant temperature sensor uses a thermistor to control signal voltage to ECM. The ECM applies a reference voltage on signal wire to sensor. When engine is cold, sensor resistance is high, and ECM receives a high signal voltage.

As engine coolant warms, sensor resistance becomes less, and voltage signal drops. At normal engine operating temperature, voltage will be about 1.5-2.0 volts at ECM terminal C10.

Coolant temperature is one of the inputs used by ECU to control the following:

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.

This step checks if code is caused by hard failure or intermittent condition. Code 15 will set if signal voltage indicates coolant temperature is less that -35°F (-37°C) for one minute.
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 determines if there is a wiring problem or defective ECM. If ground wire is open, there may also be a Code 33 stored.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code 15, Coolant Temperature Sensor (Temperature Too Low): Notes

Use the Manual With the Right Hardware

When to See a Mechanic