P0118 – Engine Coolant Temperature (ECT) Sensor Circuit High means the ECM/PCM has detected an abnormally high voltage signal from the coolant-temperature sensor. A high voltage reading indicates the sensor is reporting an unrealistically cold engine temperature or the circuit is open. When this happens, the ECM enriches fuel unnecessarily, causing rough running, increased fuel consumption, and hard starting. This guide explains the meaning of P0118, common symptoms, causes, diagnosis steps, and reliable repair options.
What Does P0118 Mean?
P0118 is a generic powertrain OBD-II code stored when the ECM detects that the Engine Coolant Temperature sensor signal is higher than expected—typically indicating a voltage above 4.9V. Since the ECT sensor is a thermistor, high voltage means the ECM believes the engine is extremely cold or that the circuit is open.
The ECM immediately enters a failsafe strategy, often using a default coolant temperature reading. This can affect fuel trims, ignition timing, idle control, and cooling-fan operation, making the code important to diagnose quickly.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Engine / Cooling / Fuel Management
- Difficulty Level: Easy–Moderate
- Estimated Repair Cost: €40–€250
- Last Updated: 28 November 2025
Real-World Example / Field Notes
A compact sedan arrived with poor fuel economy and high idle. Scanning showed P0118, and live data displayed an unrealistic –40°C coolant temperature. The technician found the ECT sensor connector was broken, leaving the circuit open. Installing a new connector and reseating the sensor resolved the issue instantly.
Another case involved a pickup truck that stalled when warm. The ECT sensor’s internal thermistor had failed, causing intermittent voltage spikes. Replacing the sensor and topping up coolant restored normal engine performance.
Symptoms of P0118 – Engine Coolant Temp Sensor Circuit High
- High idle speed: ECM thinks engine is cold and adds extra fuel.
- Poor fuel economy: Excess enrichment increases consumption.
- Hard starting when warm: Incorrect temperature input alters fuel delivery.
- Black smoke: Rich mixture caused by false cold reading.
- Cooling fans running constantly: Failsafe mode activates cooling system.
- Check engine light: P0118 stored, sometimes with fuel-trim codes.
Common Causes of P0118
Most Common Causes
- Open circuit in ECT sensor wiring
- Disconnected or damaged ECT sensor connector
- Faulty Engine Coolant Temperature sensor
- Corroded terminals causing high resistance
- Broken thermistor inside sensor
Less Common Causes
- Low coolant level creating air pockets around the sensor
- Short-to-voltage inside the wiring harness
- Faulty ECM/PCM (rare)
- Aftermarket sensors with incorrect resistance curve
- Coolant contamination causing poor sensor contact
Diagnosis: Step-by-Step Guide
P0118 requires verifying wiring integrity, sensor resistance, and proper voltage reference from the ECM.
Tools You’ll Need
OBD-II scanner, multimeter, IR thermometer, wiring diagrams, coolant tester, and basic hand tools.
- Check coolant level: Low coolant can expose the sensor tip, causing false readings.
- Review live data: Look for unrealistic temps (–40°C or –30°F indicates an open circuit).
- Inspect connector and wiring: Look for corrosion, damaged pins, or broken locking tabs.
- Measure reference voltage: With key ON, check for ~5V at the sensor connector.
- Check ground circuit: Ensure proper continuity and low resistance.
- Measure ECT sensor resistance: Compare to temperature-resistance chart.
- Warm engine and retest: Resistance should drop smoothly as temperature rises.
- Check for shorts or opens: Perform continuity tests between ECM connector and sensor.
- Use IR thermometer: Compare actual coolant temperature with scan-tool data.
- Replace sensor if out of range: Clear codes and confirm normal readings.
Pro Tip: If the scan tool shows a fixed –40°C reading, the ECT sensor circuit is almost always open—start with connector and wiring checks.
Possible Fixes & Repair Costs
- Replace ECT sensor: €20–€70
- Repair/replace wiring or connector: €30–€120
- Top up or bleed coolant system: €20–€60
- Replace thermostat (if temp reading unstable): €40–€120
- Replace ECM/PCM (rare): €200–€400+
Can I Still Drive With P0118?
You can drive short distances, but performance will suffer. The engine may run rich, overcool, or idle poorly. Long-term driving risks catalytic-converter damage due to excessive fuel. If cooling fans run constantly, overheating could occur in traffic. Repair P0118 promptly to avoid additional problems.
Related Codes
- P0129 – Barometric Pressure Too Low
- P0127 – Intake Air Temperature Too High
- P0126 – Insufficient Coolant Temperature for Stable Operation
- P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0124 – Throttle/Pedal Position Sensor/Switch “A” Circuit Intermittent
- P0123 – Throttle/Pedal Position Sensor/Switch “A” Circuit High
- P0122 – Throttle/Pedal Position Sensor/Switch “A” Circuit Low
- P0121 – Throttle/Pedal Position Sensor/Switch “A” Circuit Range/Performance
- P0120 – Throttle/Pedal Position Sensor/Switch “A” Circuit
- P0119 – Engine Coolant Temperature Circuit Intermittent
Key Takeaways
- P0118 means the ECT sensor voltage is too high, usually from an open circuit.
- Most problems involve wiring, connectors, or a failed sensor.
- ECM enters failsafe mode, causing poor drivability and rich running.
- Fixing wiring or replacing the ECT sensor often resolves the fault quickly.
FAQ
What temperature does the ECM see when P0118 is active?
Usually –40°C (–40°F), the default value when the circuit is open.
Is the engine at risk of overheating?
Indirectly, yes—fans may run constantly or fail to operate correctly, depending on failsafe strategy.
Do I need to replace the thermostat?
Only if the temperature data is inconsistent or if the engine truly runs colder than normal.
Can a loose connector cause P0118?
Absolutely. A disconnected or corroded connector is one of the most common causes.
Is P0118 serious?
It affects fueling and cooling-fan operation, so it should be repaired quickly, even though the car may still run.