DTC P0499 OBDII denotes that the PCM (Powertrain Control Module) has encountered pressure in the Evaporative Emissions Control System (EVAP), which exceeds the limit set by the manufacturer.
The EVAP system limits vapors from escaping into the atmosphere. Sometimes this system opens the purge valve which draws in fresh air needed by itself. If the EVAP System has not prescribed a purge condition and senses that there is a high-voltage pressure, which should not be there during this no-purge condition. This will establish that there is a malfunction in the EVAP System’s vent valve or its control circuit.