Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Code 15, Coolant Temp Sensor (Temperature Too Low): Notes
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, 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 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 than -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 circuit No. 452 is open, there may also be a Code 33 stored.
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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.
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