P0118

Engine Coolant Temperature Sensor Circuit 1 High Input

P0118 is a generic OBD-II powertrain diagnostic trouble code: Engine Coolant Temperature Sensor Circuit 1 High Input. It is logged by the engine control unit when the coolant monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0118
Group
Powertrain
System
Coolant
Severity
Warning (MIL on, possible limp mode)
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What P0118 means

P0118 is an SAE generic powertrain code set when the Engine Control Module (ECM/PCM) detects an abnormally high voltage signal on the Engine Coolant Temperature (ECT) Sensor 1 circuit. Under normal operation the ECT sensor is a negative temperature coefficient (NTC) thermistor: as coolant temperature rises, resistance falls and the signal voltage drops. A high-voltage reading therefore tells the ECM the coolant is implausibly cold — typically below −38 °F / −39 °C — which indicates an open circuit, a break in the sensor ground path, or an internally failed sensor rather than a genuine temperature condition.

The ECM relies on the ECT signal for fuel trim, ignition timing, idle speed, cooling fan control, and cold-start enrichment strategies. When P0118 is active the module substitutes a default (cold) temperature value, causing the engine to run rich, fans to operate continuously, and the AC compressor to be inhibited. P0118 is the high-side companion to P0117 (circuit low / sensor shorted to ground) and shares the same sensor and wiring harness.

In most calibrations the MIL illuminates on the second consecutive drive cycle that confirms the fault. Depending on vehicle make and the substitute strategy applied, the engine may enter a reduced-power or limited-operation mode; continued driving risks masking an actual overheat condition because the temperature gauge and fan logic are both compromised.

Common causes

Most-frequently reported root causes when P0118 is logged.

  • 1
    Faulty ECT sensor (internal open circuit causing abnormally high resistance / high voltage output)
  • 2
    Open circuit or broken wire in the ECT signal or ground wire
  • 3
    Corroded, backed-out, or damaged connector terminals at the ECT sensor
  • 4
    Short to positive voltage on the ECT signal circuit
  • 5
    Poor ground continuity on the ECT sensor reference ground circuit
  • 6
    Failed or stuck-open thermostat causing genuinely low coolant temperature that trips the circuit-high threshold
  • 7
    Low coolant level exposing the sensor tip to air rather than coolant
  • 8
    Defective ECM/PCM (rare; only after all wiring and sensor tests pass)

Symptoms drivers notice

Check Engine Light (MIL) illuminated
Engine runs rich with poor fuel economy and possible black exhaust smoke
Hard cold start or extended cranking
Cooling fans run continuously regardless of actual coolant temperature
Inoperative or erratic coolant temperature gauge reading
AC compressor disabled or intermittent

How to diagnose P0118

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve and record all stored DTCs and freeze-frame data; note any related codes such as P0117, P0125, or thermostat codes
  2. 2
    With a scan tool displaying live ECT data, compare the reported coolant temperature against ambient air temperature when the engine is fully cold — they should agree within ~5 °F / 3 °C; a reading near −40 °F confirms circuit open
  3. 3
    Visually inspect the ECT sensor connector and wiring for corrosion, pushed-back pins, chafing, or breaks; repair any damage found
  4. 4
    Disconnect the ECT sensor connector and measure signal-circuit voltage at the harness side — expect the ECM reference voltage (typically 5 V); then jumper the two connector terminals together and verify the scan tool reading rises toward ~285 °F / 140 °C, confirming ECM and wiring are functional
  5. 5
    Measure ECT sensor resistance across its terminals at known coolant temperatures and compare against manufacturer specification tables (e.g., ~2,000–3,000 Ω at 68 °F / 20 °C); an infinite reading confirms internal sensor failure — replace the sensor
  6. 6
    Check cooling system condition: verify coolant level, inspect for a stuck-open thermostat (engine takes excessively long to reach normal operating temperature), and confirm the system holds pressure
  7. 7
    Clear the code after all repairs and perform two complete drive cycles with a cold start to confirm the fault does not return before returning the vehicle to service

Related powertrain codes

Frequently asked questions

Can I keep driving with a P0118 code active?

It is not advisable. While the engine may run, the ECM is substituting a default cold-temperature value. This masks real overheat conditions — the temperature gauge and cooling fan logic are unreliable — which risks blown head gaskets or a warped cylinder head if actual overheating occurs undetected.

Is P0118 always caused by a bad ECT sensor?

Not always. The code means the circuit voltage is too high, which is most often an open-circuit fault: broken wire, corroded connector, or a sensor whose internal resistance has gone infinite. A wiring or connector fault is found at least as often as a failed sensor, so inspect the harness before condemning the sensor.

What is the difference between P0118 and P0117?

They are circuit-range opposites on the same sensor. P0117 (circuit low) means voltage is abnormally low, indicating the signal wire is shorted to ground or the sensor resistance is near zero. P0118 (circuit high) means voltage is abnormally high, indicating an open circuit or short to the reference voltage supply.

Will a faulty thermostat cause P0118?

Yes, in some cases. If the thermostat is stuck open, coolant may genuinely stay very cold in cold ambient conditions and trip the circuit-high detection threshold. The ECM expects the engine to warm up within a certain number of minutes; if it does not, a plausibility fault can set P0118 or the companion P0128. Replacing the thermostat resolves this variant.

Disabling P0118 in software

RaceTune can permanently disable P0118 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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