FAOSC (Rear Fuel Trim) Monitor
As the front UEGO sensor ages and gets exposed to contaminants, it can develop a rich or lean bias in its transfer function. The rear bias control (also called FAOSC - Fore/Aft Oxygen Sensor Control) system is designed to compensate for any of these bias shifts (offsets) using the downstream HO2S sensor. The "FAOS" monitor looks for any bias shifts at the stoichiometric point of the front UEGO sensor lambda curve. If the UEGO has developed a bias beyond the point for which it can be compensated for, lean (P2096, P2098) or rich (P2097, P2099) fault codes will be set.
UEGO "FAOS MONITOR" OPERATION:
| DTCs | P2096 - Post catalyst fuel trim system too lean (Bank 1) P2097 - Post catalyst fuel trim system too rich (Bank 1) P2098 - Post catalyst fuel trim system too lean (Bank 2) P2099 - Post catalyst fuel trim system too rich (Bank 2) |
| Monitor execution | Continuous while in closed loop fuel |
| Monitor Sequence | > 30 seconds time in lack of movement test, > 30 seconds time in lack of switch test |
| Sensors OK | ECT, IAT, MAF, MAP, VSS, TP, ETC, FRP, FVR, DPFE EGR, VCT, VMV/EVMV, CVS, CPV, EVAPSV, FTP, CKP, CMP, ignition coils, injectors, no misfire DTCs, no system failures affecting fuel, no EVAP gross leak failure, UEGO heaters OK, rear HO2S heaters OK, no "lack of switching" malfunction, no "lack of movement" malfunction, no UEGO circuit malfunction, no rear stream 2 HO2S circuit malfunction, no rear stream 2 HO2S functional DTCs, no rear stream 2 HO2S response rate malfunction. |
| Monitoring Duration | 5 seconds to register a malfunction |
TYPICAL UEGO "FAOS MONITOR" ENTRY CONDITIONS:
| Entry Conditions | Minimum | Maximum |
| Closed loop stoich fuel control | ||
| Time since engine start | 20 seconds | |
| Engine Coolant Temp | 160 °F | 250 °F |
| Time since entering closed loop fuel | 20 seconds | |
| Fuel Level | 15% | |
| Short Term Fuel Trim Range | -13% | 18% |
| Air mass range | 2 lbm/min | 8 lbm/min |
| Learning conditions stability time (based on air mass) | 15 seconds | |
| Injector fuel pulse width (not at minimum clip) | 650 usec | |
| Inferred HO2S 2 Heated Tip Temperature | 1100 °F | |
| No excessive movement between currently utilized long term fuel trim cells (1 = complete change from one cell to adjacent cell) | 0.5 | |
| UEGO sensor within +/- 2 % from the fuel control target | ||
| UEGO ASIC not in recalibration mode | ||
| Stream1 UEGO response test not running | ||
| Intrusive UEGO catalyst monitor not running | ||
| Not performing intrusive UEGO Lack-of-Movement fuel control defib | ||
| No air passing through during valve overlap (scavenging). | ||
| Battery Voltage | 11.0 Volts | 18.0 Volts |
TYPICAL UEGO "FAOS MONITOR" MALFUNCTION THRESHOLDS:
| >= 5 seconds since reaching the FAOSC lean or rich limits while system bias maturity is met. Lean malfunction: -0.083 rear bias trim limit Rich malfunction: 0.087 rear bias trim limit |