P0118 Code — Symptoms of a Faulty Engine Coolant Temperature Sensor

The P0118 trouble code indicates that the Engine Coolant Temperature (ECT) sensor is sending a higher-than-expected voltage signal to the engine control module (ECM).

The ECT sensor measures the temperature of the engine coolant and helps the ECM adjust:

• fuel injection

• ignition timing

• cooling fan operation

• engine warm-up strategy

When the ECM detects that the ECT sensor signal voltage is too high, it stores the code:

P0118 — Engine Coolant Temperature Sensor Circuit High Input.

This usually means the ECM believes the engine coolant temperature is extremely cold, even when it is not.

🔎 What Does Code P0118 Mean?

The engine coolant temperature sensor is a thermistor that changes resistance depending on coolant temperature.

Typical behavior:

• Cold engine → higher resistance

• Hot engine → lower resistance

If the ECM receives voltage outside the expected range, it triggers the P0118 code.

This often happens when:

• the sensor circuit is open

• wiring is damaged

• the sensor has failed

⚠️ Common Symptoms of P0118

Drivers may notice several symptoms when this code appears.

Typical symptoms include:

• Check Engine Light

• poor fuel economy

• rough idle

• hard starting when warm

• cooling fans running constantly

• incorrect temperature reading on dashboard

In some vehicles, the engine may run too rich, using more fuel than necessary.

🔧 Most Common Causes of P0118

Several issues can trigger the P0118 code.

1️⃣ Faulty Coolant Temperature Sensor

The most common cause is a failed ECT sensor that sends incorrect signals.

2️⃣ Open Circuit in Wiring

Broken wires or disconnected connectors can cause the ECM to read maximum voltage.

3️⃣ Corroded Sensor Connector

Moisture and corrosion may affect the electrical connection.

4️⃣ Low Coolant Level

Low coolant may cause incorrect temperature readings in some cases.

5️⃣ Engine Control Module Problems (Rare)

In rare situations the ECM may misinterpret the sensor signal.

💰 P0118 Repair Cost

Repair costs depend on the underlying problem.

Typical repair costs include:

In many cases, replacing the ECT sensor solves the issue.

🚗 Can You Drive With Code P0118?

The vehicle may still be drivable, but driving with this code is not recommended for long periods.

Possible issues include:

⚠ poor fuel economy
⚠ incorrect engine temperature readings
⚠ cooling fans running continuously
⚠ potential engine overheating

The problem should be diagnosed and repaired soon.

🔎 How Mechanics Diagnose P0118

Professional diagnosis usually includes:

✔ scanning live temperature data
✔ testing sensor resistance
✔ inspecting wiring and connectors
✔ checking coolant level and cooling system

Mechanics may also verify the thermostat and cooling fan operation.

🏁 Final Thoughts

The P0118 code indicates that the engine coolant temperature sensor is sending a voltage signal that is too high.

Common causes include:

• faulty coolant temperature sensor

• damaged wiring

• corroded connectors

• cooling system issues

Fixing the issue early can restore accurate engine temperature monitoring and proper engine performance.