General Catalyst Monitor Operation

Monitor execution is once per drive cycle. Typical monitor duration is 700 seconds. In order for catalyst monitor to run, HO2S monitor must be complete and Secondary AIR and EVAP system functional with no stored DTCs. If catalyst monitor does not complete during a particular driving cycle, the already accumulated switch/signal data is retained in Keep Alive Memory (KAM) and is used during next driving cycle to allow catalyst monitor a better opportunity to complete. Rear HO2S can be located in various configurations to monitor different kinds of exhaust systems. In-line engines and many V-engines are monitored by their individual bank. A rear HO2S is used along with the front fuel control HO2S for each bank. These 2 sensors are used on an in-line engine; 4 sensors are used on a V-engine. Some V-engines have exhaust banks that combine into a single underbody catalyst. These systems are referred to as Y-pipe systems. They use only one rear HO2S along with 2 front, fuel-control HO2S. Y-pipe system uses 3 sensors in all. For Y-piped systems, 2 front HO2S signals are combined by PCM software to infer what HO2S signal would have been in front of monitored catalyst. Inferred front HO2S signal and the actual single rear HO2S signal is then used to calculate switch ratio.

Most vehicles that are part of Low Emission Vehicle (LEV) catalyst monitor phase-in will monitor less than 100 percent of catalyst volume. Often this is the first catalyst brick of catalyst system. Partial volume monitoring is done on LEV and Ultra Low Emission Vehicle (ULEV) vehicles in order to meet the 1.75 emission standard. Many applications that utilize partial-volume monitoring place rear HO2S sensor after first light-off catalyst housing, or after second catalyst housing in a 3-catalyst housing per bank system. A few applications place HO2S in middle of catalyst housing, between first and second bricks.

Some 2003 model year Partial Zero Emission Vehicles (PZEV) will utilize three sets of HO2S's. The front sensors or stream 1 (HO2S11/HO2S21) are the primary fuel control sensors. The next sensors downstream or stream 2 in the exhaust are utilized to monitor the light-off catalyst (HO2S12/HO2S22). The last sensors downstream or stream 3 in the exhaust (HO2S13/HO2S23) are utilized for very long term fuel trim in order to optimize catalyst efficiency (For Aft Oxygen Sensor Control). For addition heated oxygen sensor information, see HEATED OXYGEN SENSOR MONITOR .

Index ratios for Ethanol (Flex-fuel) vehicle vary based on the changing concentration of alcohol in fuel. The malfunction threshold typically increases as percentage of alcohol increases. For example, a malfunction threshold of 0.5 may be used at E10 (10 percent ethanol) and 0.9 may be used at E85 (85 percent ethanol). Malfunction thresholds are therefore adjusted based on percentage of alcohol in fuel.