Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.
P0118: Engine coolant temperature circuit high input
Voltage in the coolant-temperature circuit is above the range the engine computer considers credible.
What P0118 means
Voltage in the coolant-temperature circuit is above the range the engine computer considers credible. On the usual sensor circuit, the computer interprets this as an extremely cold engine. A disconnected sensor, open circuit or high-resistance connection is common, but the code alone does not identify the break.
Can I still drive?
Arrange prompt diagnosis because the computer may no longer know the true engine temperature and may use substitute fan and fuelling settings. A brief, low-load journey may be reasonable only if coolant level and actual temperature remain normal and the engine starts, idles and responds normally. Stop for a red temperature warning, overheating, steam, coolant loss, stalling or severe rough running.
Symptoms
- Engine-management light.
- Temperature reading that is implausibly low or does not move.
- Hard starting, particularly when the engine is already warm.
- Rich running, rough idle, increased fuel use or dark exhaust smoke.
- Cooling fan operating continuously or unexpectedly on some vehicles.
Common causes
- Disconnected coolant-temperature sensor.
- Broken signal or earth wire.
- Loose, corroded or spread connector terminals.
- Sensor with an internal open circuit.
- High resistance in a connection.
- Control-unit fault rarely.
Safe first checks
- When cold, verify coolant level and inspect for leaks. Save codes and freeze-frame information. Compare live coolant temperature with ambient temperature before starting; a fixed, extremely low fallback value can suggest an open circuit, although the exact figure is vehicle-specific. With the ignition off, inspect the connector and accessible harness, particularly if the cooling system was worked on recently.
Typical UK fix cost
Approximately £90–£250 for diagnosis and a common sensor, plug or accessible wiring repair. Complex loom tracing, control-unit work or separate cooling-system damage will increase the total.
Related models
This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.
Related fault codes
- P0116: Engine coolant temperature circuit range/performance
- P0117: Engine coolant temperature circuit low input
- P0120: Throttle/pedal position sensor or switch “A” circuit
- P0121: Throttle/pedal position sensor or switch “A” range/performance
Sources
- Toyota service information — P0117/P0118 monitor and fault paths
- Mitsubishi service information — P0118 diagnostic procedure
- HELLA Tech World — testing a coolant-temperature sensor
- ClickMechanic — UK coolant-temperature sensor replacement guide
A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.