Engine Coolant Temperature Sensor Circuit High Input
The P0118 code indicates that the Engine Control Module (ECM) has detected a voltage signal from the Engine Coolant Temperature (ECT) sensor that is higher than the expected range, typically above 4.9 volts. This usually means the sensor circuit is open, shorted to voltage, or the sensor itself has failed. The ECT sensor is a thermistor that changes resistance with temperature; a high voltage reading corresponds to a very low resistance, which can cause the ECM to think the engine is extremely cold, leading to rich fuel mixture and potential drivability issues.
1. Most Common Causes
- Faulty engine coolant temperature (ECT) sensor (internal resistance out of spec)
- Short circuit to ground in the ECT sensor wiring
- Short circuit between the signal wire and the 5V reference voltage wire
- Corroded, oxidized, or damaged pins in the ECT sensor connector
- Engine control module (ECM/PCM) failure (rare but possible)
- Poor ground reference for the sensor or ECM
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Hard starting, especially when cold
- Poor fuel economy
- Engine running rich (black smoke from exhaust)
- Engine overheating or running too cool
- Rough idle or stalling
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the ECT sensor connector and wiring for damage, corrosion, or loose connections.
Step 3: Using a multimeter, check the ECT sensor resistance at the sensor terminals (compare to manufacturer specs for given temperature).
Step 4: Check for reference voltage (typically 5V) and ground at the sensor connector with the ignition on.
Step 5: If sensor and wiring are okay, test the ECM input by backprobing the signal wire and comparing to sensor output.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances with P0118. The ECM may use a default coolant temperature value, which can cause poor engine performance, increased emissions, and potential engine damage if overheating occurs. Have it diagnosed and repaired promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for parts and labor. A new ECT sensor costs around $20-$50, and labor is usually 0.5-1 hour. Wiring repairs may add to the cost.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.