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P0118 — Engine Coolant Temperature (ECT) Sensor — Circuit High Input

Moderate

Quick answer

P0118 means the ECT sensor’s the signal is stuck high — typically a short to voltage, a broken ground or reference wire, or a failed sensor. A lying ECT makes the computer fuel for an engine temperature that isn’t real: hard cold starts, rich running (and the fuel smell that comes with it), radiator fans locked on, or overheating that never triggers the fans. Check the connector and wiring before buying a sensor — for this variant of the code, wiring is the most common answer.

What it means

The engine coolant temperature (ect) sensor tells the engine computer how hot the engine’s coolant is — which drives cold-start enrichment, fan control, and timing. P0118 sets when the signal is stuck high — typically a short to voltage, a broken ground or reference wire, or a failed sensor for long enough that the computer stops trusting it.

Circuit codes describe electrical behavior, which puts the wiring, the connector, and the sensor on equal footing as suspects. The cheapest of the three fails most often.

While the signal is untrusted, the computer substitutes a safe default value. The engine runs, but on assumptions instead of measurements — that’s the drivability change you feel.

P0118 symptoms: what you'll notice

  • Hard starting on cold mornings — long cranking before the engine catches.
  • Radiator fans running constantly, even with a cold engine — or never coming on at all.
  • A temperature gauge that reads wrong, jumps around, or stays pinned on cold.
  • A rich fuel smell and worse economy from the engine over-fueling.
  • In the worst case, real overheating, because the fans never get the signal to turn on.

Common causes

Ordered from most to least likely.

  1. 1.

    Wiring damage (chafe, break, melted insulation)

    A short to voltage or broken ground pins the signal high.

  2. 2.

    Corroded, loose, or backed-out connector pins

    Unplug and inspect both halves under good light.

  3. 3.

    Failed ECT sensor

    Confirm with measurements before replacing.

  4. 4.

    Low coolant or air pocket at the sensor

    A sensor reading air instead of coolant reports nonsense — check level first.

  5. 5.

    Lost 5V reference or sensor ground (where applicable)

    If several sensors fault together, suspect a shared reference circuit rather than coincidence.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Read the freeze frame

    Note when P0118 sets — cold start, warm idle, under load, over bumps. The conditions narrow the cause dramatically, especially for intermittent faults.

  2. 2 Inspect connector and harness

    Unplug the sensor; check for corrosion, bent or spread pins, and chafed insulation along the harness run. Re-seat firmly. This free step resolves a remarkable share of circuit codes.

  3. 3 Watch it in live data

    Stone-cold, ECT should match IAT and ambient. From start, it should climb smoothly to operating temperature (~85–105°C) with no jumps or dropouts.

  4. 4 Verify with an infrared thermometer

    Point an IR thermometer at the thermostat housing and compare with the scanner. Several degrees of disagreement condemns the sensor; agreement points back at wiring.

  5. 5 Wiggle-test if intermittent

    Engine running, data live: gently flex the harness and tap the sensor while watching the reading. A glitch you can provoke is a fault you can find.

  6. 6 Replace with a quality part

    If measurements condemn the sensor, buy OEM or a reputable brand — bargain sensors re-set these codes often enough to cost more in time than they save in money.

Parts & tools you may need

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Frequently asked questions

What does code P0118 mean?
P0118 means the ECT sensor’s the signal is stuck high — typically a short to voltage, a broken ground or reference wire, or a failed sensor. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.
Can I drive with P0118?
Briefly — but a falsely-cold reading runs the engine rich enough to foul plugs and stress the converter, and a falsely-hot/fan failure can let it genuinely overheat. Watch the gauge.
Is it the sensor or the wiring?
For this variant, lean wiring: stuck-low, stuck-high, and intermittent signatures are circuit behaviors. Inspect and measure before buying the sensor.
Why did the code return after a new sensor?
Because the circuit, not the sensor, was the fault — or the replacement was low quality. Re-do the wiring inspection the first repair skipped.
What does the computer do meanwhile?
It substitutes a default value and keeps the engine running on assumptions. Functional, but you pay in drivability and fuel until the real measurement comes back.
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