P0456 on a GMC: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — a very small fuel vapor leak won't affect the engine, but fix it before an emissions test.

What P0456 means on a GMC

On a GMC, P0456 means the computer found a very small leak in the fuel vapor system. A fuel cap that isn't fully tightened is the usual cause, and it's one of the most common EVAP codes on GMC trucks.

Is P0456 common on GMCs?

P0456 shows up on Sierras, Yukons, and Acadias, often right after a fuel stop. When the cap isn't the fix, the vent valve at the charcoal canister and the vapor hoses underneath are the usual suspects.

Commonly reported on: Sierra, Yukon, Acadia, Terrain.

Symptoms

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

Common causes

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

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	$50–150
Replace the canister vent valve if it is the leak source	Yes	$50–150