The air/fuel ratio is constantly monitored by the Powertrain Control Module (PCM) through Oxygen (O2) Sensors. This sensor detects the oxygen levels that are present in the exhaust gas and then transmits them to the PCM.
The O2 Sensor generates a voltage reading which is used by the PCM to regulate the air/fuel ratio and maintain it to a stoichiometric ratio. In this way, the catalytic converter (TWC) can purify the exhaust gas more efficiently.
Storing the OBD2 trouble code P2198 means the PCM has found an excessively rich air/fuel ratio. It means there is more fuel than excess air. When this happens, the PCM cannot regulate the mixture. This code refers specifically to Oxygen Sensor number 1 which is on bank 2.