P0456 on a Mercedes-Benz: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — this is a tiny emissions leak, not an engine problem, but fix it before an emissions test.

What P0456 means on a Mercedes-Benz

On a Mercedes-Benz, P0456 tells you the engine computer found a very small leak in the fuel vapor system. The system pressurizes and watches for pressure loss, so even a weak filler cap seal can set it.

Is P0456 common on Mercedes-Benzs?

P0456 shows up fairly often on Mercedes models like the C-Class and E-Class, usually after the fuel cap has aged or wasn't closed tightly. A smoke test is the standard way to locate the tiny leak when the cap isn't the problem.

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

Symptoms

Check engine light is on
No change in how the car drives
Occasional faint fuel smell

Common causes

most common: Fuel filler cap not fully tightened or worn seal
common: Small leak in an EVAP hose or O-ring
common: Leaking EVAP purge valve
less common: Tiny crack in the charcoal canister or filler neck

How to fix it

Fix	DIY?	Typical cost
Retighten the fuel cap until it clicks, then clear the code	Yes	$0–30
Have a smoke test performed to find the exact leak	Mechanic	$60–150
Replace the EVAP purge valve if it is the leak source	Mechanic	$80–220