P0118 — Engine Coolant Temperature (ECT) Sensor Circuit High

Category

Powertrain → Sensors (Engine Temperature Monitoring)

What it means (plain English)

The ECT sensor monitors engine coolant temperature and reports to the ECM, which uses this data for fuel delivery, glow plug timing, fan operation, and emissions control.

A P0118 code means the ECM sees an abnormally high voltage signal from the ECT sensor. Since these are thermistors (resistance decreases as temperature increases), a high voltage signal usually indicates the ECM thinks the coolant is extremely cold (often −40°F or below).

Symptoms

  • Check Engine Light (MIL) illuminated
  • Extended cranking, hard starting in cold conditions
  • Poor fuel economy (ECM runs engine “rich” longer)
  • Excess smoke from exhaust
  • Cooling fans may run continuously or fail to engage properly
  • Rough idle until warmed up
  • Glow plug timing issues on cold starts

Priority Level

Medium–High —
An incorrect coolant reading won’t usually strand the vehicle, but it causes hard starts, wasted fuel, emissions issues, and potential overheating if cooling fans are affected.

Common Causes

  • Unplugged or disconnected ECT sensor
  • Open circuit in ECT sensor wiring
  • Corroded or damaged connector pins
  • Failed ECT sensor (internal open circuit)
  • Low coolant level (rare, but can cause poor sensor contact)
  • Faulty ECM input (rare)

How Pros Diagnose It (step-by-step)

  1. Scan tool data — compare ECT reading to ambient temperature. If it shows −40°F, suspect open circuit.
  2. Visual inspection — check ECT sensor and connector near thermostat housing.
  3. Wiring check — verify 5V reference and ground are present.
  4. Resistance test — compare sensor resistance to temperature spec chart.
  5. Substitute known-good sensor if wiring tests good.
  6. Check coolant level and flow (low coolant can sometimes trigger ECT codes).

Likely Fixes

  • Reconnect or repair ECT sensor wiring
  • Replace faulty ECT sensor
  • Repair corroded/damaged connector
  • Refill coolant if low
  • Replace ECM only if confirmed (last resort)

Related/Companion Codes

  • P0115 — ECT Sensor Circuit Malfunction
  • P0116 — ECT Range/Performance
  • P0117 — ECT Sensor Circuit Low
  • P0125 — Insufficient Coolant Temperature for Closed Loop
  • P0217 — Engine Overtemperature Condition

Tech Notes (Quick Hits)

  • Duramax LB7/LMM — common for corroded connectors near thermostat housing.
  • Cummins ISB/ISX — harness rub-through near ECT wiring is a known cause.
  • Powerstroke 6.0L/6.4L/6.7L — often throws this code when ECT connector is left unplugged after service.
  • Always compare ECT reading vs Intake Air Temp (IAT) — they should be close on a cold engine.

Severity: Medium–High — Repair Soon
A P0118 code means your engine’s coolant temperature sensor is reporting a faulty high voltage signal. The ECM thinks the engine is much colder than it really is, leading to hard starts, excess fuel use, and poor emissions. Fixing the sensor or wiring ensures proper starting, cooling, and fuel economy.

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