Air Fuel Ratio Imbalance Monitor - O2 Sensor Monitor
The Air Fuel Imbalance Monitor is designed to monitor the cylinder-to-cylinder air fuel imbalance per engine bank. When an Air Fuel (A/F) imbalance is present, the front UEGO signal becomes noisier. The monitor uses the high frequency component from the UEGO signal as an indicator of A/F imbalance. The UEGO signal may be either voltage or lambda, depending on the sensor ASIC being used.
"Hash" is the difference between two consecutive front UEGO voltage samples. The UEGO signal is monitored continuously and a differential or "hash" value is continuously calculated. When the hash is below a threshold, it is indicative of normal operation. If the hash exceeds the threshold, an A/F imbalance is assumed. The hash is accumulated (summed) over a calibratable period, typically 50 engine cycles. This period is referred to as a window.
Monitor completion requires each speed load cell to acquire a calibratable number of data windows. There are up to three speed load cells. Typically, each cell must acquire 30 data windows. In addition, the total number of data windows for all the cells must be larger than a calibration value that is normally set to 20% larger than the sum of the minimum number of windows for each cell. For example, if three speed load windows are used, and each requires a minimum of 30 data windows, then the calibration for the total number of windows would be set to 3*30*1.2 = 108 data windows.
When the monitor completes, an A/Fuel imbalance index is calculated. The monitor index is defined as the ratio of the failed rpm windows over the total rpm windows required to complete monitor. An index is calculated for each of the speed load cells being used. The final index for each engine bank is a weighted average of each of the three cell indices. If the monitor imbalance ratio index exceeds the threshold value, an A/F imbalance DTC is set.
| DTCs | P219A - Bank 1 Air-Fuel Ratio Imbalance P219B - Bank 2 Air-Fuel Ratio Imbalance |
| Monitor execution | Once per driving cycle during closed loop |
| Monitor Sequence | Monitor runs after fuel monitor has adapted |
| Sensors OK | ECT, IAT, MAF, VSS, TP, ETC, FRP, DPFE EGR, VCT, VMV/EVMV, CVS, 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 | Time to complete monitor ranges from 300 to 700 seconds |
| Entry Condition | Minimum | Maximum |
| Closed Loop Fuel Control | ||
| Engine Air Mass | 2 lb/min | 10 lb/min |
| Engine RPM Cell 0 | 1250 rpm | 3000 rpm |
| Engine RPM Cell 1 | 1700 rpm | 2100 rpm |
| Engine RPM Cell 2 | 2100 rpm | 3400 rpm |
| Engine Load Cell 0 | 40% | 70% |
| Engine Load Cell 1 | 50% | 80% |
| Engine Load Cell 2 | 60% | 90% |
| Engine Coolant Temp | 150 °F | 250 °F |
| Intake Air Temp | 20 °F | 150 °F |
| Throttle Position Rate of Change | 0.122 v/100 msec | |
| Fuel percentage from purge | 40% | |
| Fuel Level | 15% | |
| Fuel monitor has adapted | ||
| No purge on/off transition | ||
| Fuel type leaning is complete (FFV only) |
| Imbalance Ratio Bank 1 >.75 Imbalance Ratio Bank 2 >.75 |
| Monitor ID | Test ID | Description | |
| $81 | $80 | Bank 1 imbalance-ratio and max. limit (P219A/P219B) | unitless |
| $82 | $80 | Bank 2 imbalance-ratio and max. limit (P219A/P219B) | unitless |