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 Sensor Temp To Low: Notes
The coolant temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on circuit No. 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine coolant temperature the sensor inputs to the ECM are used to control the following:
- Fuel delivery.
- Electronic spark timing (EST).
- Cooling fan.
- Converter clutch (TCC).
- Idle (IAC).
- Checks to see if code was set as a result of hard failure or intermittent condition. Code will set if engine has been running for more than 50 seconds. Signal voltage indicates a coolant temperature below -22°F (-30°C).
- This test simulates conditions for a Code 14. If the ECM recognizes the grounded circuit (low voltage), and displays a high temperature, the ECM and wiring are okay.
- This test will determine if there is a wiring problem or a faulty ECM. If circuit No. 452 is open, there may also be a Code 21 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.