P0118 medium Severity

P0118: Engine Coolant Temperature Sensor Circuit High Input

The engine control module receives an implausibly high voltage from the engine coolant temperature sensor circuit leading to incorrect temperature readings, which can cause rich fuel mix, hard starting, poor fuel economy, rough idle, and potential overheating risk.

Overview

P0118 engine coolant temperature sensor circuit high input; high voltage signal from ECT sensor; causes, diagnostics, and repair steps

Common Symptoms

  • Check engine light illuminated
  • Poor fuel economy
  • Rough idle or misfires
  • Hard starting or stalling
  • Constant cooling fan operation or perceived overheating

Most Common Causes

Faulty Engine Coolant Temperature (ECT) sensor high

Sensor internal thermistor failure causing high/abnormal voltage readings.

Open circuit or break in wiring harness high

Open or disconnected wiring leads to high reference voltage at PCM interpreted as high input.

Corroded or damaged sensor connector medium

Connector pins corrosion or poor contact increases resistance and voltage reading errors.

Low or contaminated coolant affecting sensor reading medium

Air around sensor tip or contamination can produce inaccurate sensor signals.

Faulty PCM/ECM (rare) low

Control module internal reference voltage issue misinterprets normal voltage.

Diagnostic Steps

  1. Retrieve codes and review live data with OBD‑II scanner
    Expected: ECT temperature reading unrealistically low (e.g., -40°C) while engine is warm
    Interpretation: Indicative of high voltage/open circuit condition.
  2. Visually inspect ECT sensor connector and wiring
    Expected: Check for corrosion, broken pins, damaged insulation, or loose connections
    Interpretation: Poor connections or damaged wiring likely cause.
  3. Check reference voltage at sensor connector with key on
    Expected: Approximately 5 volts from PCM
    Interpretation: Presence validates supply; absence indicates wiring or PCM issue.
  4. Test sensor resistance relative to temperature using multimeter
    Expected: Resistance decreases with increasing temperature per spec chart
    Interpretation: Out‑of‑range resistance points to faulty sensor.
  5. Check ground circuit continuity between PCM and sensor
    Expected: Low resistance to ground
    Interpretation: High resistance/ open ground indicates wiring fault.

Repair Solutions

Replace Engine Coolant Temperature (ECT) sensor easy
Estimated Cost: $25 - $450

Part cost varies by vehicle; sensor replacement often resolves the high voltage signal.

Repair or replace wiring and connectors medium
Estimated Cost: $30 - $350

Repair broken wires, replace corroded connectors, improve grounding.

Top up or replace coolant and bleed cooling system easy
Estimated Cost: $20 - $150

Ensures sensor is immersed and accurate readings are possible.

Replace PCM/ECM hard
Estimated Cost: $900 - $1200

Only after eliminating sensor and wiring causes; rare scenario.

Common Misdiagnoses

Avoid these common diagnostic mistakes:

  • Assuming thermostat failure triggers P0118 without testing sensor circuit
  • Replacing PCM without verifying sensor and wiring integrity

Frequently Asked Questions

Technical definition of P0118 OBD-II Code

Technically, the P0118 code represents Engine Coolant Temperature Sensor Circuit High Input. It usually points to a fault within the Sensors.

Should I stop driving if I have P0118?

You should treat P0118 as a medium issue. The engine control module receives an implausibly high voltage from the engine coolant temperature sensor circuit leading to incorrect temperature readings, which can cause rich fuel mix, hard starting, poor fuel economy, rough idle, and potential overheating risk.

What are the symptoms of P0118?

The most typical signs are: Check engine light illuminated, Poor fuel economy, Rough idle or misfires.

Potential causes for OBDII code P0118

Potential culprits include: Faulty Engine Coolant Temperature (ECT) sensor, Open circuit or break in wiring harness, Corroded or damaged sensor connector. A proper diagnosis is recommended.

What is the price difference for fixing P0118?

The cost varies significantly based on the root cause. For example, replacing a Top up or replace coolant and bleed cooling system typically costs between $20 and $150, while fixing a Replace PCM/ECM can range from $900 to $1200.

Does P0118 affect Ford, Toyota, or Honda?

This is a universal code. You might see P0118 on Ford, Chevrolet, Toyota, Honda, Nissan, or any other OBD-II vehicle.

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