The Three Way Catalyst (TWC) aims to improve tailpipe emissions by delivering oxygen to the exhaust gases. It also converts hydrocarbons, carbon monoxide, as well as nitrogen oxides. This converts it into water vapor, carbon dioxide, and nitrogen, respectively.
The Powertrain Control Module (PCM) ensures that the TWC is efficient enough. To do this, it uses an Oxygen Sensor (O2) which is located on the back of the Catalytic Converter (TWC). The O2 sensor has a single purpose, it must change more slowly than the front Oxygen (O2) sensors.
If the PCM detects that the post-catalyst O2 is performing the changeover too quickly, it is interpreted as a malfunction of the TWC. Therefore, the DTC code P0422 is set.