Engine Coolant Temperature Sensor Circuit High
The engine coolant temperature sensor is reading a high voltage signal.
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What it means (plain English)
The sensor that measures the temperature of the engine coolant is sending a voltage that is too high, which can cause the engine to run poorly, or risk overheating because the computer thinks it's cooler than it is.
What the computer is actually seeing
The engine control module (ECM) sees a voltage reading from the coolant temperature sensor that exceeds the expected range, typically above 4.5 volts for high signal conditions.
What a healthy reading looks like
Normal operating voltage for the coolant temperature sensor is generally between 0.5 and 4.5 volts, corresponding to a temperature range from -40°F to 260°F.
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. Check the coolant level and ensure it's at the correct fill level. Decision: Is the coolant level low? If yes, top it up and retest. If no, proceed.
- 22. Inspect the wiring and connector at the coolant temperature sensor for any visible damage or corrosion. Decision: Is there damage? If yes, repair and retest. If no, proceed.
- 33. Using a multimeter, measure the voltage at the connector of the coolant temperature sensor with the engine running. Decision: Is the voltage above 4.5 volts? If yes, replace the sensor and retest. If no, proceed.
- 44. Check the ground connection for the coolant temperature sensor. Decision: Is the ground good? If no, repair the ground and retest. If yes, proceed.
- 55. Clear the fault code and run the engine to see if the code returns. Decision: Does the code return? If yes, consider replacing the ECM.
- 66. If all else fails, refer to manufacturer specifications for further testing procedures.
Common causes
- Faulty engine coolant temperature sensor
- Short to voltage in the sensor circuit
- Damaged wiring or poor connections
- Corrosion in the connectors
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
While it may be drivable, running an engine with a faulty coolant temperature sensor risks overheating, so it’s best to address the problem soon.
Will the code come back after repair?
If the underlying issue is resolved, such as a bad sensor or wiring, the code should not return.
Why does this code sometimes pair with SPN 1203?
SPN 1203 indicates a specific problem with the coolant temperature sensor circuit, which often relates to the same sensor issues as SPN 1599.
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