What does it mean?
OBD II fault code P0499 is a generic code that is defined as “Evaporative Emission System Vent Valve Control Circuit High”, and is set when the PCM (Powertrain Control Module) detects that a part of the EVAP (Evaporative Emission Control System) is not working within design parameters. Note that this code refers specifically to the electrical control circuit of the Evaporative Emission System Vent Valve.
The function of the EVAP system is to capture fuel vapors before they can escape into the atmosphere. In a fully functional system, fuel vapors are collected and passed through a charcoal canister in which the activated charcoal stores the vapors, before being passed to the inlet tract through a network of hoses to be burned as part of the air/fuel mixture when operational conditions allow this to happen.
The vent valve works in conjunction with the charcoal canister to achieve this, with the function of the vent valve being to allow atmospheric air to enter the EVAP system to help displace fuel vapors as the vapor is purged from the system to be combusted in the engine. On almost all applications, the vent valve is normally open, and the application of a 12-volt circuit current closes the valve when the PCM needs to pressurize the EVAP system to test for leaks.
Problems in the control circuit or a failure of the vent valve itself will store a trouble code and illuminate a warning light.