P0118 Honda Civic: Engine Coolant Temperature Sensor Circuit High

Quick Answer: What is P0118?

On a Honda Civic, the P0118 code means the Engine Control Module (ECM) is detecting a voltage signal above 4.91V from the coolant sensor. This tells the computer the engine is at -40°F (-40°C), even if it's boiling hot. This is usually caused by an open circuit (broken wire) or a dead ECT sensor.

Severity: HIGH. Because the computer thinks the engine is freezing, it will dump excessive fuel into the cylinders, causing rich running and potential engine flooding.

Common Symptoms

Difficulty Starting: Especially when the engine is already warm.
Cooling Fans Running Constantly: The ECM runs them as a fail-safe to prevent overheating.
Poor Fuel Economy: Significant drop in MPG due to rich fuel mixture.
Black Smoke: May be visible from the tailpipe.

Most Likely Causes

Faulty ECT Sensor (80%): Internal open circuit in the thermistor.
Broken Connector: Common on Honda Civic due to engine vibrations.
Corroded Wiring: High resistance in the 5V reference or ground wires.
Low Coolant/Air Pockets: Preventing the sensor from touching the liquid.

Detailed Repair Cost Breakdown

Estimates for Honda Civic models (All generations) in the US market:

Component / Task	Aftermarket Part	OEM Honda Part	Labor Time	Difficulty
ECT Sensor Replacement	$15 - $35	$55 - $80	0.5 - 1.0 hr	Very Easy (DIY)
Connector/Pigtail Repair	$10 - $20	$45 - $60	0.5 - 1.0 hr	Easy
Diagnostic (Shop Fee)	N/A	N/A	1.0 hr	Standard

How to Fix P0118 on a Honda Civic

Step 1: Visual Inspection

Locate the ECT sensor (usually on the cylinder head near the distributor or thermostat housing). Check for broken wires or a loose connector. If the plastic clip is broken, the sensor will lose contact and trigger P0118.

Step 2: The "Jump" Test

Unplug the sensor and use a small wire or paperclip to jump the two terminals in the connector. Look at your OBD2 live data. If the temperature jumps to 280°F+ (140°C+), the wiring and ECM are fine, and the sensor is definitely dead.

Step 3: Replace the Sensor

If the jump test confirms a bad sensor, unscrew the old unit and quickly install the new one to minimize coolant loss. Top off the radiator with Genuine Honda Type 2 coolant and bleed the air from the system.