P0118 — ECT Sensor Circuit High Input Explained

Quick Answer

• What it means: The Engine Coolant Temperature (ECT) sensor is sending an unusually high voltage signal to the Powertrain Control Module (PCM), indicating a potential problem with the sensor or its circuit.
• Severity: Moderate. While you might be able to drive home, continuous driving with this code can lead to engine overheating, poor fuel economy, and potential engine damage.
• Most common cause: A faulty ECT sensor itself.

What the P0118 code means

The P0118 diagnostic trouble code stands for "Engine Coolant Temperature Circuit High Input." In simpler terms, the vehicle's computer (PCM or ECM) is receiving a voltage signal from the ECT sensor that is higher than what it expects under normal operating conditions. The ECT sensor measures the temperature of the engine coolant and sends this information to the PCM, which uses it to adjust fuel injection, ignition timing, and cooling fan operation. A high input signal typically indicates that the PCM is seeing a very low temperature or an open circuit within the sensor or its wiring.

Common symptoms

• Check Engine Light illuminated
• Engine running rich (excessive fuel consumption, black smoke from exhaust)
• Hard starting, especially when cold
• Engine overheating (though the gauge might read cold if the sensor is faulty)
• Poor fuel economy
• Erratic engine temperature gauge readings (may fluctuate or stay at cold)
• Cooling fans running constantly

Common causes

1. Faulty Engine Coolant Temperature (ECT) sensor: This is the most prevalent cause. The internal resistance of the sensor can fail, causing it to send an incorrect high voltage signal.
2. Open circuit in the ECT sensor wiring: A break or corrosion in the wires leading to the ECT sensor can interrupt the signal, causing a high input reading.
3. Poor electrical connection at the ECT sensor: Loose or corroded connectors can prevent a proper signal from reaching the PCM.
4. Low engine coolant level: While not a direct electrical issue, extremely low coolant can cause the sensor to read incorrectly high or low temperatures, though usually it would be low.
5. Faulty PCM (rare): In very rare cases, the PCM itself could be malfunctioning and misinterpreting the sensor's input.

How a technician diagnoses it

1. Verify the complaint and visual inspection: The technician will confirm the Check Engine Light is on and perform a visual inspection of the ECT sensor, its wiring, and connectors for obvious damage, corrosion, or signs of rodent damage.
2. Scan for Diagnostic Trouble Codes (DTCs): A scan tool will be used to confirm the P0118 code and check for any other related codes. Freeze frame data will be reviewed to understand engine conditions at the time the code was set.
3. Monitor ECT sensor readings: Using a scan tool, the technician will observe the ECT sensor's live data. A normal ECT sensor reading should increase steadily as the engine warms up. If the reading is stuck at an extremely low temperature (e.g., -40°F/-40°C) or extremely high voltage (e.g., 5V), it indicates a problem.
4. Test ECT sensor resistance: The technician will disconnect the ECT sensor and test its resistance using a multimeter. The resistance should decrease as the sensor warms up. Comparing the readings to manufacturer specifications will determine if the sensor is within an acceptable range.
5. Check ECT sensor circuit voltage: With the sensor disconnected, the technician will check the voltage at the sensor connector using a multimeter. One wire should have a 5-volt reference signal from the PCM, and the other should be a ground. If the 5V reference is missing or incorrect, it indicates a wiring issue.
6. Test for continuity in the wiring: The technician will check for continuity between the ECT sensor connector and the PCM connector to identify any open circuits or shorts in the wiring.
7. Inspect PCM connections: As a last resort, if all other checks reveal no issues, the PCM connectors will be inspected for corrosion or damage.

Typical repair cost

The cost to repair a P0118 code can vary depending on the exact cause and the vehicle make and model.

• ECT Sensor Replacement:
  • Parts: $20 - $100
  • Labor: $60 - $150
  • Total: $80 - $250
• Wiring Repair:
  • Parts: $5 - $20 (for connectors/wire)
  • Labor: $80 - $200 (depending on complexity)
  • Total: $85 - $220

Can I keep driving with this code?

It is not recommended to keep driving for an extended period with a P0118 code. While your car might seem to run, the PCM will likely be operating in a "default" mode, which can lead to:

• Significantly reduced fuel economy.
• Increased harmful emissions.
• Potential for engine overheating due to incorrect fan operation or fuel mixture.
• Catalytic converter damage if the engine runs excessively rich for too long.

Driving for a short distance to a repair shop might be acceptable, but prolonged driving should be avoided.

Related codes

• P0115: Engine Coolant Temperature Sensor Circuit Malfunction
• P0116: Engine Coolant Temperature Sensor Circuit Range/Performance Problem
• P0117: Engine Coolant Temperature Sensor Circuit Low Input
• P0119: Engine Coolant Temperature Sensor Circuit Intermittent
• P0125: Insufficient Coolant Temperature for Closed Loop Fuel Control

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