Engine Coolant Temperature Sensor 1 Circuit Low
Indicates low voltage from the engine coolant temperature sensor.
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What it means (plain English)
The engine coolant temperature sensor is sending a voltage reading that is lower than expected, making the computer think the engine is cooler than it really is; this can cause poor performance and increased emissions.
What the computer is actually seeing
The engine control module (ECM) sees a voltage from the coolant temperature sensor that is below 0.2 volts, indicating a faulty sensor or circuit.
What a healthy reading looks like
The expected voltage range from a normal functioning coolant temperature sensor should be between 0.5 to 4.5 volts at operating temperature (around 190°F/88°C).
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 the wiring and connectors from the coolant temperature sensor for any visible damage or corrosion. Decision: If damaged, repair or replace; otherwise, move to step 2.
- 22. Measure the voltage at the coolant temperature sensor connector with the key on, engine off. You should see 0.5 to 4.5 volts. Decision: If volts are out of range, replace the sensor; if within range, go to step 3.
- 33. Start the engine and monitor the coolant temperature gauge or data scanner. Ensure it rises with engine temperature. Decision: If it does not rise appropriately, check for proper coolant level and potential leaks; if it does rise correctly, ECM may be faulty.
Common causes
- Faulty coolant temperature sensor
- Damaged wiring or connectors
- Cooling system leaks
- Erroneous ECM logic errors
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's moderate severity; you can drive but should fix it soon to avoid performance issues or increased emissions.
Will this code come back after clearing?
If the fault is fixed, clearing the code should keep it off, but if the problem remains, it'll likely come back.
Why does this code seem to occur with others like SPN 1209?
Codes often overlap due to shared components or circuits; fixing one can resolve multiple related issues.
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