PCM uses a closed loop air/fuel metering system. The PCM monitors all vehicle HO2S signal voltages to control fuel delivery. Fuel delivery is controlled only when in Closed Loop Operation. When fuel is added, the HO2S will increase its voltage signal. When the PCM sees the HO2S voltage increase passed a calculated point, fuel delivery will be reduced. The PCM will increase and decrease fuel delivery to keep the HO2S crossing 500mv, and Air Fuel Mixture near 14.7:1. The PCM monitors the overall changes in fuel delivery by looking at the long-term and the short-term fuel trim values. The ideal fuel trim value is around 0%. The PCM will add fuel when the HO2S signal is indicating a lean condition (below 0%). PCM will reduce fuel delivery when fuel trim is above 0%. The DTC relevant to fuel trim will be set when the amount reaches excessive levels because of a lean or rich condition trend.