P0115 Honda Civic: Engine Coolant Temp (ECT) Sensor Malfunction

Quick Answer: What is P0115?

The P0115 code on a Honda Civic indicates that the Engine Control Module (ECM) is receiving an irrational signal from the Engine Coolant Temperature (ECT) sensor. Essentially, the computer doesn't know how hot or cold the engine is, which messes up the fuel mixture and cooling fan operation.

Severity: HIGH. If the sensor tells the computer the engine is cold when it's actually overheating, you could blow a head gasket. Fix this code immediately.

Symptoms

Difficulty Starting: Especially when the engine is already warm.
Excessive Fuel Consumption: The car stays in "rich mode" longer than necessary.
Radiator Fans: Fans may run constantly or not at all.
Black Smoke: Visible from the exhaust due to a rich air-fuel mixture.

Most Likely Causes

Faulty ECT Sensor (85%): Internal failure of the thermistor.
Corroded Connector: Common in Hondas due to moisture near the thermostat housing.
Wiring Short: Broken or frayed wires in the sensor harness.
Low Coolant: Air pockets preventing the sensor from getting a reading.

Specific Repair Cost Breakdown

Pricing for Honda parts is generally affordable, but labor varies depending on the shop location:

Repair Component	Aftermarket Part	OEM Honda Part	Labor Time	Total (Est.)
ECT Sensor	$18 - $30	$55 - $75	0.5 - 1.0 hr	$80 - $180
Sensor Connector/Pigtail	$12 - $20	$35 - $50	1.0 hr	$100 - $160
Coolant Flush & Refill	$15 (Fluid)	$25 (Genuine)	0.5 hr	$60 - $100

How to Fix P0115

1. Check the Connector First

Locate the sensor near the thermostat housing. Unplug it and look for green corrosion. Clean it with electrical contact cleaner and see if the code clears.

2. Test Sensor Resistance

Use a multimeter to check the ohms of the sensor. A healthy Honda sensor should show high resistance when cold and low resistance as the engine warms up. If the ohms don't change, the sensor is dead.

3. Replace the Sensor

With the engine cold, unscrew the old sensor. Have the new one ready to swap in quickly to avoid losing too much coolant. Tighten to 18 ft-lbs and top off the radiator.