P0118 — Coolant Temp Sensor High Input

What does P0118 mean?

P0118 is set when the engine control module reads the engine coolant temperature sensor signal voltage above the calibrated maximum — typically a reading that would represent an unrealistically cold coolant temperature (often −40 °F / −40 °C). This is the open-circuit fingerprint for the ECT sensor: a disconnected sensor, a broken signal wire, or a sensor failed open internally.

What are the symptoms of P0118?

• Check Engine Light is illuminated
• Temperature gauge reads zero or stays cold
• Difficult cold start with rich-running symptoms
• Cooling fans may run constantly (ECM thinks coolant is dangerously cold)
• Slightly reduced fuel economy
• Cold-running engine doesn't warm up properly per the gauge
• Possible hard start when the engine is hot (ECM commands cold-start enrichment)

What causes P0118?

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0118?

In most cases a vehicle stays drivable for short trips with P0118 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0118

1. Read ECT temperature with a scan tool

   A working ECT reads close to ambient air temperature when the engine has been off long enough to cool. A reading of "−40 °F" or "−40 °C" is the classic open-circuit fingerprint.

   Tools: Scan tool with ECT PID

2. Inspect the connector and visible wiring

   Locate the ECT sensor (usually threaded into the intake manifold or thermostat housing). Unplug, inspect for corrosion, water, bent pins. Re-seat and clear the code — if it stays clear, the connector was the problem.

   Tools: Electrical contact cleaner, Flashlight

3. Bench-test the ECT sensor resistance

   Remove the sensor (place a rag — coolant will drip). Measure resistance across its terminals at room temperature. Compare to the service manual — most NTC ECTs read 2.0–4.5 kΩ at 68 °F (20 °C). Infinite resistance means the sensor is open and needs replacement.

   Tools: Multimeter, Service spec sheet, Coolant catch and replacement

4. Verify signal-wire continuity to the PCM

   With the ECT disconnected, check continuity from the sensor's signal wire to the corresponding PCM pin. Open circuit confirms a broken wire somewhere along the path. Visual inspection of the harness — particularly where it routes near hot exhaust or moving accessories — often reveals the chafe point.

   Tools: Multimeter, Wiring diagram

5. Jumper-test the signal wire

   With the ECT disconnected, briefly jumper the signal wire to ground and observe the scan tool — the reading should swing from "very cold" to "very hot." No swing points at the wiring or PCM input rather than the sensor.

   Tools: Jumper wire, Scan tool

How do I fix P0118?

• Reconnect the ECT sensor connector
• Replace the ECT sensor with an OEM part
• Repair the broken ECT signal wire
• Clean corroded connector pins

Pick your vehicle for a P0118 diagnosis

Select your exact year, make, and model below to get a diagnostic guide tuned to your vehicle's known failure patterns and TSBs.

Related diagnostic codes

• P0125 — Coolant Too Cold for Closed-Loop Fuel Control
• P0128 — Engine Not Reaching Operating Temperature
• P0420 — Catalyst Efficiency Below Threshold
• P0300 — Random or Multiple Cylinder Misfire
• P0171 — Fuel System Too Lean (Bank 1)
• P0440 — EVAP System Malfunction (Generic)