What does it mean?
The emissions profile of your car includes CO2 (carbon dioxide), which is directly related to fuel economy, and others, such as NOX, CO, and HC (oxides of nitrogen, carbon monoxide, and unburned hydrocarbons). Additionally, even when the car isn’t running, it may also be responsible for evaporative emissions, that is, fuel vapors. The warmer it is, the faster fuel evaporates, which is great inside your engine, but not so great in the environment.
The Evaporative Emissions Control System, typically referred to simply as the “EVAP” system, is designed to capture fuel vapors and route them to the engine, where they can be burned. The EVAP system includes an activated-charcoal canister, for storing fuel vapors, tubing connecting the canister to the fuel tank and the engine intake.
At least two valves are used to control the storage and flow of fuel vapors, such as the purge valve and the vent valve, and pressure sensors are used to verify system operation. The ECM (engine control module) controls and monitors the EVAP system. While purging the charcoal canister, for example, the ECM will command the purge and vent valves to open. Engine intake vacuum creates a partial vacuum in the EVAP system, while fresh air enters the vent valve, flushing out fuel vapors to be burned.
At the same time, using voltage drops and pressure readings, the ECM monitors EVAP system function. If the ECM detects a problem in the EVAP system, it will set a DTC (diagnostic trouble code) and illuminate the MIL (malfunction indicator lamp). In case the ECM detects a problem in the vent valve circuit, it will set DTC P0498, “Evaporative Emission (EVAP) System Vent Valve Control Circuit Low,” and illuminate the MIL.
Depending on the automaker, the vent valve may be called the canister close valve (CCV) or vent shut valve.