P0118 on a Mercedes-Benz: 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 Mercedes-Benz

On a Mercedes-Benz, P0118 means a coolant temperature sensor is sending a high-voltage signal that the computer reads as the engine being very cold. Depending on the model, the sensor is mounted on the cylinder head or the radiator, and an open circuit from a failed sensor or broken wire is the usual cause. The computer then runs the engine rich as if it were cold.

Is P0118 common on Mercedes-Benzs?

P0118 appears on C-Class and E-Class models with age. On some late-1990s and early-2000s Mercedes, the engine wiring insulation is known to break down, which can cause open circuits like this one. Sensor failures are also part of normal aging.

Commonly reported on: C-Class, E-Class, S-Class, GLC.

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: Deteriorated wiring insulation on older models
less common: Corroded or loose connector

How to fix it

Fix	DIY?	Typical cost
Replace the coolant temperature sensor	Yes	$40–120
Repair the sensor wiring	Mechanic	$100–300
Have a shop diagnose the circuit	Mechanic	$120–180

Frequently asked questions

Can I drive my Mercedes with a P0118 code?

Yes. The engine will run rich and use more fuel, but it is not an emergency. Fix the circuit soon to restore fuel economy.

Why does P0118 happen on Mercedes-Benz models?

A failed coolant temperature sensor is the most common cause. On older models, engine wiring with degrading insulation can also open the sensor circuit.

How much does it cost to fix P0118 on a Mercedes-Benz?

A replacement sensor is about $40 to $120, plus labor if a shop does it. Wiring repairs run about $100 to $300.