P0118 — Engine Coolant Temperature Sensor 1 Circuit High
P0118 sets when the ECT sensor signal voltage is above the calibrated maximum, indicating an open circuit or failed sensor. Actual temperature comparison prevents unnecessary sensor replacement when the wiring is the actual cause. An open ECT sensor usually causes long cranking times and rich running.
Fault Overview
Open signal wire, failed ECT sensor, poor connection, and short to power are the primary causes. Compare scan-tool temperature to actual temperature with an infrared thermometer. Check the connector at the thermostat housing carefully.
Engine & Platform Coverage
Symptoms
- •P0118
- •ECT circuit high
- •coolant temperature sensor high voltage
- •ECT open
Possible Causes
- 1.Open signal wire
- 2.Failed ECT sensor
- 3.Poor connection
- 4.Short to power
Wiring & Circuit Notes
ECT sensor connector and wiring harness. Check for corrosion at the thermostat housing connector.
DVOM Specs & Sensor Tests
ECT voltage vs temperature chart. Sensor resistance (NTC). Reference voltage and ground.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Measure ECT Voltage: Measure ECT signal voltage. A reading near 5 V indicates an open circuit.
- 2Step 2 — Compare Scan Data to Actual Temperature: Compare scan-tool temperature to actual temperature with an infrared thermometer.
- 3Step 3 — Disconnect Sensor and Observe Voltage: Disconnect the sensor and observe the voltage. If voltage stays near 5 V, the wiring is open.
- 4Step 4 — Measure Resistance of the Sensor: Measure resistance of the ECT sensor (NTC thermistor) and compare to the temperature vs resistance chart.
- 5Step 5 — Check Wiring and Replace Sensor if Needed: Check wiring continuity and replace the ECT sensor if the circuit is proven good but the sensor is failed.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Repair open wiring $30–150; replace ECT sensor $30–150.
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.