In order to provide the best possible combination of drivability, fuel economy and emission control, the ECM uses a closed loop air/fuel metering system. The ECM monitors the HO2S signal voltage and adjusts fuel delivery. When the fuel delivery is based on the HO2S it is known as Closed Loop Fuel Control. As more fuel is injected, the HO2S will increase voltage. When the ECM see the HO2S voltage rise, it will reduce the more of fuel injected. As the fuel delivery changes to keep the HO2S crossing 500mv, the Air Fuel Mixture is near 14.7:1. The ECM allows us to monitor 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 ECM will add fuel when the HO2S signal is indicating a lean condition. When the fuel trim values that are above 0%, the system is having to add more fuel to keep the mixture at 14.7:1. . The ECM will reduce fuel when the HO2S signal is indicating a rich condition. Reduction in fuel is indicated by fuel trim values that are below 0%. The DTC relevant to fuel trim will be set when the amount reaches excessive levels because of a lean or rich condition.

Checking the fuel trime value at idle under detecting condition, if its average exceeds the limit over certain period, ECM sets P2189.