Fuel trim is the action the PCM takes to achieve the correct air/fuel mixture which allows the engine to operate normally. When the engine load increases, the O2 Sensor number 2 constantly prompts the PCM to make adjustments to the mixture for maximum engine efficiency.
The diagnostic code P0173 OBDII is set when the PCM (Powertrain Control Module) cannot maintain the stoichiometric air/fuel mixture. You may receive a fuel leveling request from the post-catalytic Oxygen (O2) Sensor, and that request exceeds the ability of the PCM to configure the fuel setting to meet the required setting level for a while.