EVAP — Evaporative Emission System

Gasoline evaporates. The vapors that rise off the fuel in the tank are hydrocarbons — a pollutant. The EVAP system captures those fuel vapors and stores them in a charcoal canister instead of allowing them to escape into the atmosphere. When driving conditions allow, the PCM opens a purge valve that draws those stored vapors from the canister into the intake manifold where the engine burns them.

Charcoal canister — a container filled with activated charcoal that absorbs and stores fuel vapors. Canister purge valve — a PCM-controlled valve in the line between the canister and the intake manifold. When open, engine vacuum draws stored vapors from the canister. Canister vent valve — controls the air inlet to the canister for testing purposes. The gas cap — seals the fuel tank. A loose, missing, or damaged gas cap is the single most common cause of EVAP codes.

P0440 through P0457 are EVAP system codes. The most common customer concern is a check engine light with an EVAP small leak code — P0456 — or large leak code — P0455. The PCM tests the EVAP system by sealing it and monitoring for pressure changes. A leak anywhere in the system — from the gas cap to the canister to the purge line — triggers a code. A smoke machine is the best tool for finding EVAP leaks — introduce smoke into the EVAP system and watch for smoke escaping at the leak point.