Coolant Temperature Sensor 1
Indicates a problem with the coolant temperature sensor signal.
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What it means (plain English)
The computer is not getting a proper temperature reading from the coolant temperature sensor; it thinks the engine is either too hot or too cold.
What the computer is actually seeing
The Engine Control Module (ECM) expects a voltage signal from the coolant temperature sensor that typically ranges from 0.5V (at -40°C) to 4.5V (at 125°C). If the signal is out of this range, it triggers a fault.
What a healthy reading looks like
Voltage from the coolant temperature sensor should be 0.5V to 4.5V depending on temperature.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Inspect wiring and connectors to the coolant temperature sensor for damage or corrosion. If found, repair or replace.
- 22. Test the coolant temperature sensor with a multimeter. Check that it produces the expected voltage when heated or cooled. If it doesn’t, replace the sensor.
- 33. Check coolant levels and look for leaks. If low, top off and recheck the sensor operation. If problems persist, investigate further.
- 44. Monitor the ECM data stream with a scan tool while the engine is running. Confirm that the coolant temperature readings are fluctuating with temperature changes. If readings are static, suspect ECM or additional sensor issues.
Common causes
- Faulty coolant temperature sensor
- Wiring issues (shorts or opens)
- Corroded or loose connectors
- Coolant leaks causing sensor exposure to air
- Faulty Engine Control Module (ECM)
Typical repair cost
$100-$300
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's best to fix this soon to avoid potential overheating or poor engine performance.
Will this code come back?
If the underlying issue (like a bad sensor or wiring) is not fixed, yes.
Why does this code pair with SPN 1294?
SPN 1294 indicates a different sensor in the same system, such as a wiring or signal issue affecting multiple sensors.
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