The Fuel System Monitor is an on-board strategy designed to monitor the fuel trim system. The fuel control system uses fuel trim tables stored in the PCM's Keep Alive Random Access Memory (RAM) to compensate for variability in fuel system components due to normal wear and aging. During closed-loop vehicle operation, the fuel trim strategy learns the corrections needed to correct a "biased" rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has two means of adapting; a Long Term Fuel Trim and a Short Term Fuel Trim. Long Term relies on the fuel trim tables and Short Term refers to the desired air/fuel ratio parameter "LAMBSE". Both are described in greater detail in this section under Powertrain Control Software, Fuel Trim. Input from the ECT, IAT, and MAF sensors is required to activate the fuel trim system, which in turn activates the Fuel System Monitor. Once activated, the Fuel System Monitor looks for the fuel trim tables to reach the adaptive clip and LAMBSE to exceed a calibrated limit. The Fuel System Monitor will store the appropriate DTC when a fault is detected as described below.

1. The heated oxygen sensor (HO2S) detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.

2. A correction factor is added to the fuel injector pulsewidth calculation according to the Long and Short Term Fuel Trims as needed to compensate for variations in the fuel system.

3. When deviation in the parameter LAMBSE increases, air/fuel control suffers and emissions increase. When LAMBSE exceeds a calibrated limit and the fuel trim table has clipped, the Fuel System Monitor sets a Diagnostic Trouble Code (DTC) as follows:
   
   The DTCs associated with the monitor detecting a lean shift in fuel system operation are DTCs P0171 and P0174.
   
   The DTCs associated with the monitor detecting a rich shift in fuel system operation are DTCs P0172 and P0175.

4. The MIL is activated after a fault is detected on two consecutive drive cycles.
   
   
   
   Figure 11: Fuel System Monitor