P0118: Engine Coolant Temperature Sensor 1 Circuit High
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0118 means
The engine coolant temperature (ECT) sensor signal voltage is too high, so the ECM sees an engine that is impossibly cold.
What sets it
The ECT sensor works like the IAT: a thermistor whose resistance falls as temperature rises. An open circuit pulls the signal toward the full reference voltage, which reads as extremely cold, and sets P0118. Because ECT affects fueling, idle, the cooling fans and transmission behavior, many ECMs switch on the cooling fans and use default values when this code is active.
Symptoms
- Hard starting or black smoke (rich cold-start fueling)
- Cooling fans running constantly (failsafe on many vehicles)
- Temperature gauge reading cold or not moving
- Poor fuel economy
Is it safe to drive?
General guidance, not a guarantee: Use caution. Check the coolant level and the actual engine temperature first. A low coolant level can leave the sensor in air, and an engine that is actually overheating needs to be stopped. With coolant full and the fans running, short careful trips are usually possible. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Open sensor
- Unplugged or corroded connector
- Open signal or ground wire
- Low coolant leaving the sensor in steam or air: gives false readings
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- With the engine cold, check the coolant level first.
- Read the ECT PID. A reading stuck at the scan tool's lowest value (often -40) suggests an open circuit (general guideline).
- Compare ECT with the actual engine temperature from an infrared thermometer near the sensor, or with IAT on a fully cold-soaked engine.
- Inspect the connector for corrosion and damage.
- Unplug the sensor and briefly jumper signal to ground per accepted practice. A jump to very hot means the wiring and ECM are fine. Only if the manufacturer's procedure allows. Some ECMs and circuits can be damaged by jumpering; use a fused jumper or a test resistor where specified.
- Check the ground circuit for voltage drop, since many sensors share a ground.
- Replace the sensor using the correct procedure and bleed air from the cooling system if coolant is lost.
Bench note: When the coolant is low the sensor can read cold while the engine is actually hot. Check the level before you trust the reading.
How-to guides for this code
Related codes
FAQ
Why are my cooling fans always on?
Many ECMs run the fans continuously as a failsafe when they can't trust the coolant temperature reading.
Can low coolant cause P0118?
It can cause false readings if the sensor tip sits in air or steam. Always check the level first.
P0118 vs P0128: what's the difference?
P0118 is an electrical circuit-high fault on the sensor. P0128 is a warm-up rationality check that usually points to the thermostat.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- California Code of Regulations, 13 CCR 1968.2, OBD II requirements (via Cornell LII)
- Generic code title cross-check: obd-codes.com generic code list
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.