P0118 on a 2017 Honda Civic

What does P0118 mean on a 2017 Honda Civic?

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.

Symptoms on a 2017 Honda Civic

• 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)

Likely causes on a 2017 Honda Civic

1. Disconnected or unlatched ECT sensor connector Most common
   Estimated repair: $0– $50
2. Broken or chafed ECT signal wire (open circuit) Common
   Estimated repair: $80– $350
3. Internally failed ECT sensor (element opened) Common
   Estimated repair: $30– $200
4. Corroded sensor connector pins Common
   Estimated repair: $30– $200
5. Damaged sensor body from coolant leak or freeze Occasional
   Estimated repair: $50– $250
6. Failed PCM signal input (rare) Rare
   Estimated repair: $400– $1,500

How to diagnose this on a 2017 Honda Civic

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

Common fixes

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

About the 2015-2019 Honda Civic

The 2015-2019 Honda Civic was commonly sold with the following powertrains: 2.0L I4, 1.5L Turbo I4, 1.8L I4. Common trims include LX, Sport, EX, Touring.

Related diagnostic codes

• P0115
• P0116
• P0117
• P0119
• P0125
• P0128