Fuel & air metering · Powertrain · generic (SAE-defined)
P0118: Engine Coolant Temperature Circuit High
P0118 means the engine coolant temperature (ECT) sensor signal voltage is higher than expected, making the engine computer think the coolant is extremely cold, often due to an open circuit.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0118 sets
As coolant cools, the engine coolant temperature (ECT) sensor's resistance rises, increasing the signal voltage the powertrain control module (PCM) reads. If the voltage rises above the highest plausible value for a set time, P0118 sets. An unplugged sensor, broken wire, open ground or failed sensor is the typical cause. Exact limits vary by manufacturer.
Common causes of P0118
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Unplugged or loose ECT connector
- Open signal or ground wire
- Failed (open) ECT sensor
- Corroded connector terminals
- PCM fault (rare)
P0118 symptoms
- Check engine light on
- Temperature gauge reading cold or not moving
- Cooling fans running constantly
- Rich running, black smoke or poor fuel economy
- Rough idle when warm
How P0118 is diagnosed
- Check coolant level and confirm actual engine temperature
- Scan for codes; live data typically shows a very cold ECT value
- Verify the connector is fully seated and undamaged
- Jumper signal to ground at the connector; a very hot reading confirms wiring is intact
- Check the ground circuit for an open
- Measure sensor resistance and replace if open
Typical fixes for P0118
- Reconnect or replace the ECT connector
- Repair open wiring
- Replace the ECT sensor
- Bleed the cooling system after replacement
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
Older definitions read "Engine Coolant Temperature Circuit High Input". Definitions can vary by manufacturer; check factory service information.
Related codes & warning lights
P0118 FAQ
What does code P0118 mean?
P0118 means the engine coolant temperature (ECT) sensor signal voltage is higher than expected, making the engine computer think the coolant is extremely cold, often due to an open circuit. The standard definition is "Engine Coolant Temperature Circuit High."
Can I drive with a P0118 code?
You can usually drive briefly, but the engine may run rich and the temperature gauge may be wrong, so it could overheat without warning. Watch for steam or loss of heat, and repair it promptly.
Why does P0118 cause black smoke?
The PCM thinks the engine is very cold and adds extra fuel for enrichment, which can make it run rich.
Can a cold climate set P0118?
Real cold weather does not normally push the reading beyond the sensor's range; the code usually points to an open circuit or failed sensor.
Keep researching
Engine guides by brand (known issues, specs)
More fuel & air metering codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.