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DTC code page

P0118: Engine Coolant Temperature Circuit High Input

Quick answer: The ECU is seeing an abnormally high-voltage ECT signal, which usually corresponds to an implausibly cold reading.

Drivers also search this fault as engine coolant temperature circuit high input, ECT high input, coolant temp sensor reads cold all the time.

Severity: medium Family: powertrain Related paths: 13
Meaning

What P0118 usually means

P0118 means the coolant temperature signal is higher than the ECU expects. Because most ECT sensors are negative-temperature-coefficient thermistors, a high-voltage signal usually means the control module thinks the engine is extremely cold. That can keep cold-enrichment active too long, hurt fuel economy, darken exhaust, and distort catalyst and thermostat monitoring.

Fast triage

Start here before chasing parts

  • Scan first: save freeze-frame and pending codes before clearing anything.
  • Confirm the complaint: compare the stored code with current drivability symptoms.
  • Use context: trims, live data, and related codes usually narrow the fault faster than guesswork.
  • Work simplest to hardest: leaks, connectors, maintenance items, and known patterns before expensive components.
Initial checks

What to check first

  • Compare the reported coolant temperature to ambient on a fully cold engine and note if it is implausibly colder than reality.
  • Inspect the sensor connector carefully because a partly disconnected plug is a classic P0118 cause.
  • Look for companion rich-running, thermostat, or catalyst warm-up codes because false-cold data often drags those in behind it.
Driving risk

Can you keep driving?

P0118 often leaves the vehicle driveable, but it can waste fuel, foul plugs, and confuse catalyst or thermostat monitors. It is worth fixing before secondary codes pile up.

Moderate urgency: This code often allows short-term driving, but the right fix usually comes faster when you diagnose it early instead of waiting for more codes.
Likely causes

Common causes behind this code

  • ECT sensor unplugged or internally open
  • Open signal or ground wire in the ECT circuit
  • Connector spread terminal or corrosion creating high resistance
  • Intermittent harness break that opens when the engine moves or warms up
  • Rare PCM fault after the sensor and circuit are proven good

Cause phrases often tied to this code: open ECT circuit, sensor unplugged, high resistance in wiring, poor connector contact, PCM sees false cold engine.

Diagnostic order

Suggested workflow

  1. Verify the engine-coolant reading on a cold soak and again after full warm-up; a false-cold signal often barely rises enough.
  2. Inspect the sensor connector fit and tug lightly on the wires to find broken strands near the pigtail.
  3. Check circuit continuity and ground integrity before replacing parts.
  4. Test the sensor resistance at known temperatures and compare it with specification.
  5. After repair, verify fuel trims, warm-up speed, and catalyst-related monitors behave more normally.
Avoid guesswork

Common mistakes

  • Replacing oxygen sensors for rich running before noticing the coolant input is stuck unrealistically cold.
  • Calling P0128 the primary fault when false-cold ECT data is the reason warm-up logic looks wrong.
  • Ignoring a connector that looks latched but has spread terminals inside.
Repair path

Practical fix guidance

  • Repair the open circuit, poor terminal tension, or failed sensor that is holding the signal high.
  • Once the ECT reading is believable again, retest any rich-running or warm-up catalyst codes before replacing unrelated parts.
  • Confirm the engine reaches normal temperature and that live data tracks reality from cold start to full warm-up.
Vehicle context

Affected brands in this MVP

Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.

Aliases and common searches

English phrases tied to P0118

Useful when the driver knows the wording but not the exact DTC yet.

  • engine coolant temperature circuit high input
  • ECT high input
  • coolant temp sensor reads cold all the time
Related search intent

Queries this page can answer naturally

  • P0118 code meaning
  • what does P0118 mean
  • coolant temp circuit high input
  • ECT sensor stuck cold
FAQ

Quick questions about P0118

Can P0118 cause rich running?

Yes. If the ECU thinks the engine is much colder than it really is, it may command excess warm-up enrichment.

Is P0118 related to P0128?

Yes. False-cold coolant data can distort warm-up logic enough to overlap with thermostat-style complaints and related codes.

What is the fastest clue for P0118?

A scan tool showing an impossibly cold engine on a vehicle that is already warm or has been sitting in mild weather is a strong clue.