P0118
Engine Coolant Temperature Sensor 1 Circuit High
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The P0118 trouble code indicates that your vehicle's Engine Control Module (ECM) has detected an abnormally high voltage reading from the Engine Coolant Temperature (ECT) sensor circuit. The ECT sensor is a critical component that monitors the temperature of your engine's coolant and sends this information to the computer, which uses it to adjust fuel mixture, ignition timing, and emission controls. When the ECM receives a voltage signal that's higher than the expected range (typically above 4.9 volts or indicating temperatures below -40°F/-40°C), it triggers the P0118 code and illuminates your Check Engine Light.
This code matters because your engine relies on accurate coolant temperature data to operate efficiently. When the ECM receives false high-voltage readings, it may incorrectly believe the engine is extremely cold and compensate by enriching the fuel mixture excessively. This can lead to poor fuel economy, rough running conditions, increased emissions, and potential long-term engine damage from running too rich. Additionally, if there's an actual cooling system problem masked by faulty sensor readings, you could risk engine overheating without proper warning.
If you receive a P0118 code, you should address it within a day or two, though it's typically safe to drive short distances to a repair facility. Start by checking your coolant level and inspecting the ECT sensor connector for obvious corrosion or damage. Most cases are resolved by replacing the relatively inexpensive coolant temperature sensor, which is a straightforward DIY repair for moderately skilled home mechanics. However, proper diagnosis should include testing the sensor resistance with a multimeter and inspecting the wiring harness for shorts or opens before replacing parts. Professional diagnosis costs $100-150, while total repairs typically range from $100-350 depending on whether you need just a sensor or wiring repairs.
Severity
Symptoms
- •Check Engine Light is illuminated
- •Engine runs rough or stalls, especially when cold
- •Poor fuel economy (10-25% decrease)
- •Hard starting or extended cranking, particularly in cold weather
- •Engine overheating or cooling fan running constantly
- •Black smoke from exhaust due to rich fuel mixture
Likely Causes
The ECT sensor itself has failed internally, sending incorrect high-voltage signals to the ECM. This is the most common cause, especially in vehicles with 80,000+ miles or 5+ years of age.
Corrosion, fraying, or poor connections in the sensor harness can create high resistance or open circuits, causing the ECM to read abnormally high voltage signals that trigger the P0118 code.
A short to voltage in the signal wire between the sensor and ECM will cause the computer to read consistently high voltage, triggering this code even if the sensor itself is functioning properly.
Though rare, internal ECM failure or damaged circuitry on the coolant temperature input can misread sensor signals. This typically only occurs after water intrusion or electrical surges.
If coolant is extremely low, the sensor may be exposed to air instead of coolant, reading abnormally high temperatures and triggering the code. Always check coolant level first during diagnosis.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the Engine Coolant Temperature (ECT) sensor ($15-60 parts)
- Repair or replace damaged wiring harness or connectors ($20-80 parts)
- Clean corroded electrical connectors with electrical contact cleaner ($5-15)
- Refill coolant to proper level and bleed air from cooling system ($15-30 parts)
- Replace Engine Control Module (ECM) if all other tests pass ($200-800 parts, typically requires professional programming)
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