P0118
UniversalEngine Coolant Temperature Circuit High Input
The engine computer has detected a high electrical input in the engine coolant temperature sensor circuit. The sensor helps the computer estimate coolant temperature and adjust engine operation. A high input can make the reported temperature seem colder than it really is, but the code alone does not confirm the actual coolant temperature or identify a failed part.
⚠️ Before You Begin (Safety First)
If the engine is overheating, steaming, or losing coolant, stop safely and shut it off; do not open a hot cooling system. If there are no overheating signs, arrange diagnosis soon, since an inaccurate temperature reading can affect engine operation.
Common Symptoms
- The check-engine light may be on, sometimes with no noticeable change in how the vehicle runs.
- Scan data may show coolant temperature colder than expected because a high circuit input can be interpreted as a low temperature.
- Starting, idle, fuel use, or emissions may be affected if the engine computer adjusts operation based on an inaccurate temperature reading.
- A temperature gauge or warning indication may be inaccurate on vehicles that use this sensor circuit for display information.
Common Causes
- An open or high-resistance connection in the sensor circuit, including a loose, corroded, or damaged connector.
- Damaged wiring or a circuit fault that causes the input signal to remain high.
- A coolant temperature sensor that is electrically faulty.
- A fault in the engine computer or its circuit is possible, but should be considered only after the sensor and wiring have been checked.
Diagnostic Steps
- Check the coolant level only when the engine is cool, and look for visible leaks or signs of overheating. Do not open a hot cooling system.
- With the engine off, inspect the coolant temperature sensor connector and nearby wiring for looseness, corrosion, damage, or contamination.
- If you have a scan tool, compare the reported coolant temperature with the engine's condition when cold and observe whether the reading changes plausibly as the engine warms. Use vehicle-specific information to interpret the readings.
- If the problem is not clear from these checks, have the sensor and circuit tested against the vehicle's wiring diagram and service specifications rather than replacing a part based on the code alone.
When to Call a Professional
Compare the coolant-temperature reading on a scan tool with the engine's actual temperature, especially when the engine is cold. Inspect the sensor connector and wiring, then test the circuit and sensor using the vehicle's wiring diagram and service specifications. A high input can result from an open circuit or poor connection as well as a sensor or wiring fault, so confirm the cause before replacing parts.
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Commonly Asked Questions
Does P0118 mean the engine is actually too hot?
No. It identifies a high electrical input in the coolant temperature sensor circuit, not a confirmed engine temperature. Check for actual overheating signs and compare scan data with the vehicle's condition.
Does this code prove the coolant temperature sensor has failed?
No. A sensor fault is one possibility, but an open circuit, poor connector contact, or damaged wiring can also produce a high input. Diagnosis should check the full circuit.
Can I drive with P0118?
The code alone does not establish whether the engine is overheating. If the vehicle shows overheating, steam, or coolant loss, stop safely and shut it off. Otherwise, arrange diagnosis soon because an inaccurate temperature signal can affect engine operation.
Why can a high input appear to mean a cold engine?
The sensor circuit's electrical signal is used to estimate temperature. Depending on the circuit design, a high input can be interpreted by the engine computer as a temperature colder than expected.
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