DTC P2099: Post Catalyst Fuel Trim System Bank 2 - High Limit: Description

One function of the Heated Oxygen (HO2S) bank 2 sensor 2 (threaded into the front exhaust pipe directly behind the cylinder 2, 4, 6 pre-converter) is to determine the amount of time for fuel trim biasing. Fuel trim biasing is the time the ECM holds a rich or lean command during idle or cruising conditions based on engine operating parameters intended to keep the HO2S bank 2 sensor 2 voltage at a constant 600 mV. As the HO2S bank 2 sensor 1 becomes active, the ECM will go into closed loop fuel control. At this time, the ECM will toggle between a rich and lean command (called perturbation) used to improve catalytic converter efficiency. Under normal steady cruise conditions, the HO2S bank 2 sensor 2 should fluctuate slightly between 500 mV and 800 mV If the ECM is commanding lean, the HO2S bank 2 sensor 2 voltage should remain steady for a certain length of time as the catalytic converter is storing oxygen. Eventually, the amount of oxygen the converter can store is overcome by the lean condition and the HO2S bank 2 sensor 2 value will eventually drop. This process works exactly opposite for the rich fuel command when the converter is releasing oxygen. Using this process, the ECM can control the voltage level out of the HO2S bank 2 sensor 2. The fuel trim bias time is designed to be centered around zero seconds. A positive time indicates the ECM is holding a rich command for longer than calibrated. A negative time indicates the ECM is holding a lean command for longer than calibrated. The ECM will continuously run a check to make sure the fuel trim bias time is within limits. DTC P2099 will set if fuel trim bias time is too far in the positive range indicating the ECM has exceeded its rich limit threshold. This indicates the HO2S bank 2 sensor 1 signal voltage has shifted up.