What P0118 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 1, places it in the fuel and air metering subsystem; the last two digits identify the specific fault.
The ECT sensor is a thermistor in the cooling system that reports engine temperature. It is one of the most influential inputs the engine computer has: it governs cold-start enrichment, idle speed, when closed-loop fuel control begins, and often when the cooling fans run.
What “Circuit High” means here
"High" means the signal or control voltage rose above the maximum the module expects. On a temperature sensor of this type (a thermistor fed from a 5 V reference through a resistor in the module), high signal voltage reads as a very low temperature, often the bottom of the scale. The usual explanations are an open circuit — an unplugged or corroded connector, a broken wire — or an open sensor.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0118:
- Hard starting, especially cold
- Black smoke or rough running when cold
- Cooling fans running constantly or not at all
- Temperature gauge reading wrong on vehicles where it shares the sensor
- Poor fuel economy
Where to start diagnosing P0118
- Confirm the coolant level first.
- On a cold engine, ECT and IAT should read close to each other and to ambient.
- Watch ECT climb as the engine warms; it should rise smoothly and settle at the normal operating temperature for the engine.
- Inspect the connector, which sits in a hot, wet area and is prone to corrosion.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0118. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Engine coolant temperature (PID 0x05) | Smooth rise to operating temperature with no jumps or flat-lining at an implausible value. |
| Intake air temperature (PID 0x0F) | Cold-start comparison point. |
| Fuel system status (PID 0x03) | Shows whether the engine has entered closed loop. |
Related codes
Codes from the same engine coolant temperature (ECT) sensor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
Further reading
Frequently Asked Questions
What does P0118 mean?
P0118 is the generic SAE J2012 code for "Engine Coolant Temperature Sensor 1 Circuit High". In plain terms, it means the engine computer has detected a "circuit high" fault on the engine coolant temperature (ECT) sensor.
Can I keep driving with P0118?
If the temperature gauge or fans are affected, watch for overheating and do not keep driving a car that is running hot.
What should I check first for P0118?
Confirm the coolant level first.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.