Air Fuel Ratio Imbalance Monitor
The air fuel ratio imbalance monitor is an on board diagnostic strategy designed to monitor the cylinder to cylinder air fuel ratio per engine bank. The air fuel ratio imbalance monitor estimates the cylinder to cylinder difference using the front heated oxygen sensor (HO2S) high frequency signal. The difference between two consecutive front HO2S signals is continuously monitored and a differential signal value is calculated. If the difference between two consecutive samples exceeds a calibrated threshold, a cylinder to cylinder deviation is estimated and the differential signal accumulation is calculated. The differential signal accumulation is calculated continuously after vehicle startup and during closed loop fuel conditions during a short calibrated RPM window. Typically the window has over 50 engine revolutions. The differential signal accumulation is then compared to a calibrated signal threshold. The counter is incremented if the threshold is exceeded. At the same time, the total RPM window counter calculates number of completed RPM windows. When the monitor completes a calibrated number of total RPM windows, the air fuel ratio imbalance index is calculated. The monitor index is a ratio of failed RPM windows over total RPM windows required to complete the monitor. If the monitor index exceeds the threshold value the test fails. The malfunction indicator lamp (MIL) is activated for air fuel ratio imbalance DTCs.