Fuel System Monitor

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 PCM Keep Alive Memory (KAM) to compensate for variability in fuel system components due to normal wear and aging. During closed-loop vehicle operation, fuel trim strategy learns corrections needed to correct a "biased" rich or lean fuel system. Correction is stored in fuel trim tables. Fuel trim has 2 means of adapting; a Long Term Fuel Trim (LONGFT) and a Short Term Fuel Trim (SHRTFT). Long term relies on fuel trim tables and short term refers to desired air/fuel ratio parameter "LAMBSE". For additional fuel trim information, see FUEL TRIM under POWERTRAIN CONTROL SOFTWARE in THEORY & OPERATION - EEC-V - GASOLINE & NGV. Input from ECT or CHT, IAT, and MAF sensors is required to activate fuel trim system, which in turn activates fuel system monitor. Once activated, fuel system monitor looks for fuel trim tables to reach adaptive clip and LAMBSE to exceed a calibrated limit. Fuel system monitor will store appropriate DTC when a fault is detected as follows.

1. Heated Oxygen Sensor (HO2S) detects presence of oxygen in exhaust and provides PCM with feedback indicating air/fuel ratio.
2. A correction factor is added to fuel injector pulse width calculation according to long and short term fuel trims as needed to compensate for variations in 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:
   • DTCs P0171 and P0174 are associated with monitor detecting a lean shift in fuel system operation.
   • DTCs P0172 and P0175 are associated with the monitor detecting a rich shift in fuel system operation.
4. MIL is activated after a fault is detected on 2 consecutive drive cycles.