P0118 on a Mazda: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

Safe to drive, but the engine may run rich and use more fuel until the sensor circuit is repaired.

What P0118 means on a Mazda

On a Mazda, P0118 means the engine coolant temperature (ECT) sensor is sending a high-voltage signal that the computer reads as the engine being very cold. The sensor sits in the coolant near the thermostat housing on most Mazdas, and an open circuit from a failed sensor or broken wire is the usual cause. The computer then adds fuel as if the engine were always cold.

Is P0118 common on Mazdas?

P0118 shows up on higher-mileage Mazda 3, Mazda 6, and CX-5 models. Connector corrosion and wires that break at the sensor are common findings, and the sensor itself fails more often as the car ages.

Commonly reported on: Mazda 3, Mazda 6, CX-5, CX-9.

Symptoms

Check engine light on
Poor fuel economy
Hard cold start
Engine runs rich

Common causes

most common: Faulty coolant temperature sensor
common: Open sensor signal wire
common: Corroded or loose connector
less common: Wiring damaged by heat or rubbing

How to fix it

Fix	DIY?	Typical cost
Replace the coolant temperature sensor	Yes	$25–70
Repair the sensor wiring or connector	Mechanic	$50–150
Have a shop diagnose the circuit	Mechanic	$80–120