P0118
Engine Coolant Temperature Sensor Circuit High Input
The engine computer is getting a coolant temperature reading that is too high or not believable. This usually means the coolant temperature sensor, its wiring, or the connector has a problem. The engine may run poorly until the issue is fixed.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $15–$80
- SHOP
- $120–$350
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Failed engine coolant temperature sensor
- Open circuit, damaged wiring, or broken connector at the ECT sensor
- Corroded, loose, or backed-out terminal in the sensor connector
Typical repair cost
DIY usually runs $15–$80. Typical shop repair lands around $120–$350, depending on the root cause.
01 / Definition
P0118 means the powertrain control module has detected a high voltage condition in the engine coolant temperature (ECT) sensor circuit. In most systems, high circuit voltage indicates the sensor is reading extremely cold or the circuit is open due to a bad sensor, damaged wiring, or poor connector contact. The PCM uses this signal for fuel control, ignition timing, cooling fan operation, and emissions strategy.
02 / Drive status
With caution. You may be able to drive short distances, but the engine can run too rich, idle poorly, or overheat without warning if the reading is wrong. Have it diagnosed soon.
03 / Symptoms
- Check engine light on
- Hard starting, especially when warm
- Rough idle or poor fuel economy
- Cooling fans running incorrectly
- Black smoke or rich-running symptoms
- Temperature gauge reading abnormally low or high
- Transmission shifting differently on some vehicles
04 / Causes
| 1 | Failed engine coolant temperature sensor | high |
|---|---|---|
| 2 | Open circuit, damaged wiring, or broken connector at the ECT sensor | high |
| 3 | Corroded, loose, or backed-out terminal in the sensor connector | medium |
| 4 | Low coolant level or air pocket affecting the sensor reading | medium |
| 5 | Faulty thermostat causing abnormal engine temperature behavior | low |
| 6 | PCM issue or reference voltage problem | low |
05 / Diagnostic sequence
- 01Check coolant level and verify the engine is not low on coolant.
- 02Inspect the ECT sensor connector for corrosion, bent pins, looseness, or coolant intrusion.
- 03Inspect the wiring harness near the thermostat housing, intake manifold, and engine front for damage or melted insulation.
- 04Read live data with a scan tool and compare the coolant temperature to the actual cold engine temperature.
- 05Unplug the ECT sensor and check whether the scan tool reading changes to an extreme value, which helps identify an open circuit.
- 06Test sensor resistance and compare it to the manufacturer temperature/resistance chart.
- 07Check for 5-volt reference and ground at the connector if the circuit design uses them.
- 08Repair wiring or replace the sensor if tests confirm a fault, then clear codes and retest.
06 / Repairs
| 1 | Top off coolant and bleed air from the cooling system if the level is low. | $10–$40 |
|---|---|---|
| 2 | Clean, repair, or replace the ECT sensor connector and wiring as needed. | $20–$120 |
| 3 | Replace the engine coolant temperature sensor if it fails resistance or live-data testing. | $15–$80 |
| 4 | Repair thermostat, cooling system, or PCM-related faults if testing shows those issues. | $60–$350 |
07 / Related codes
08 / FAQ
Can P0118 be caused by low coolant?
Yes. Low coolant or trapped air can make the sensor read incorrectly and trigger the code.
Is P0118 usually a bad sensor or wiring problem?
A bad sensor or an open wiring/connector problem are the most common causes.
Will P0118 make my car run rich?
Yes, it can. The engine computer may think the engine is colder than it really is and add too much fuel.
Can I clear P0118 without fixing it?
You can clear it, but it will likely come back if the fault is still present.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.