Storing the P2099 OBDII fault code means that the vehicle’s computer has detected an excessive amount of unburned hydrocarbon molecules in the exhaust stream downstream of the catalytic converter. On the other hand, it also means the PCM cannot correct the air/fuel mixture.
The O2 Sensor is designed to provide the Powertrain Control Module (PCM) with real-time information about the percentage of oxygen that is present in the engine exhaust compared to the environment oxygen.