Catalyst Efficiency Monitor

Catalyst Efficiency Monitor:

The Catalyst Efficiency Monitor uses an oxygen sensor before and after the catalyst to infer the Hydrocarbon (HC) efficiency based on oxygen storage capacity of the catalyst. Under normal, close-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear Heated Oxygen Sensor (HO2S) very slow and reduces the amplitude of those switches as compared to the switching frequency and amplitude of the front HO2S. As the catalyst efficiency deteriorates due to thermal and/or chemical deterioration, its ability to store oxygen declines. The post-catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude, approaching the switching frequency and amplitude of the pre-catalyst or upstream HO2S.

NOTE: The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposition on the front brick of the catalyst), not thermal deterioration.

All vehicles utilize an FTP-based (Federal Test Procedure) catalyst monitor. This simply means that the catalyst monitor must run during a standard FTP emission test. This differs from the 20-second steady state catalyst monitor used in 1994 through some 1996 vehicles. Currently, two slightly different versions of the catalyst monitor are utilized - the Switch Ratio method and the Index Ratio method. Beginning with the 2001 model year and beyond, both versions will continue to be used in subsequent model years.

Switch Ratio Method
1. In order to assess catalyst oxygen storage, the monitor counts front and rear HO2S switches during part-throttle, close-loop fuel condition after the engine is warmed-up and inferred catalyst temperature is within limits. Front switches are accumulated in up to nine different air mass regions or cells although three air mass regions is typical. Rear switches are counted in a single cell for all air mass regions. When the required number of front switches has accumulated in each cell, the total number of rear switches is divided by the total number of front switches to compute a switch ratio. A switch ratio near 0.0 indicates high oxygen storage capacity; hence high HC efficiency. A switch ratio near 1.0 indicates low oxygen storage capacity; hence low HC efficiency. If the actual switch ratio exceeds a calibrated threshold switch ratio, the catalyst is considered failed.

Inputs from Engine Coolant Temperature (ECT) or Cylinder Head Temperature (CHT) (warm engine), Intake Air Temperature (IAT) (not extreme ambient temperatures), Mass Air Flow (MAF) (greater than minimum engine load), Vehicle Speed Sensor (VSS) (within vehicle speed window) and Throttle Position (TP) (at part-throttle) are required to enable the Catalyst Efficiency Monitor.

Typical Switch Ratio Monitor Entry Conditions:
^ Part throttle with no rapid throttle transients
^ Minimum 330 seconds since start-up at 70°F (21°C)
^ Engine coolant temperature is between 170°F (76.6°C) and 230°F (110°C)
^ Intake air temperature is between 20°F (-6°C) and 180°F (82°C)
^ Engine load greater than 10%
^ Time since entering close loop is 30 seconds
^ Vehicle speed is between 5 and 70 mph (8 and 112 km/h)
^ Inferred Catalyst Mid-bed Temperature of 900°F (482°C)
^ Mass air flow is between 1 and 5 lbs/mm
^ Fuel level greater than 15%
^ EGR is between 1 and 12%

2. The Diagnostic Trouble Codes (DTCs) associated with this test are DTC P0420 (Bank 1 or Y-pipe system) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average algorithm is used for malfunction determination, up to six driving cycles may be required to illuminate the Malfunction Indicator Lamp (MIL) during normal customer driving. If Keep Alive Memory (KAM) is reset or the battery is disconnected, a malfunction will illuminate the MIL in 2 drive cycles.

Index Ratio Method
1. In order to assess catalyst oxygen storage, the catalyst monitor counts front HO2S switches during part-throttle, closed-loop fuel conditions after the engine is warmed-up and inferred catalyst temperature is within limits. Front switches are accumulated in up to three different air mass regions or cells. While catalyst monitoring entry conditions are being met, the front and rear HO2S signal lengths are continually being calculated. When the required number of front switches has accumulated in each cell, the total signal length of the rear HO2S is divided by the total signal length of the front HO2S to compute a catalyst index ratio. An index ratio near 0.0 indicates high oxygen storage capacity; hence high HC efficiency. A switch ratio near 1.0 indicates low oxygen storage capacity; hence low HC efficiency. If the actual index ratio exceeds the threshold index ratio, the catalyst is considered failed.

Inputs from ECT or CHT (warm engine), IAT (not extreme ambient temperatures), MAF (greater than minimum engine load), VSS (within vehicle speed window) and TP (at part-throttle) are required to enable the Catalyst Efficiency Monitor.

Typical Index Ratio Monitor Entry Conditions:
^ Minimum 330 seconds since start-up at 70°F (21°C)
^ Engine coolant temperature is between 170°F (76.6°C) and 230°F (110°C)
^ Intake air temperature is between 20°F (-6°C) and 180°F (82°C)
^ Time since entering close loop is 30 seconds
^ Inferred Rear HO2S sensor temperature of 900°F (482°C)
^ EGR is between 1 and 12%
^ Part throttle, maximum rate of change 0.2 volts/0.050 sec
^ Vehicle speed is between 5 and 70 mph (8 and 112 km/h)
^ Fuel level greater than 15%
^ First Air Flow Cell
^ Engine RPM 1,000 to 1,300 rpm.
^ Engine load 15 to 35%.
^ Inferred catalyst temp. 850°F (454°C) to 1,200°F (649°C).
^ Number of front O2 switches: 50.
^ Second Air Flow Cell
^ Engine RPM 1,200 to 1,500 rpm.
^ Engine load 20 to 35%.
^ Inferred catalyst temp. 900°F (482°C) to 1,250°F (677°C).
^ Number of front O2 switches: 70.
^ Third Air Flow Cell
^ Engine RPM 1,300 to 1,600 rpm.
^ Engine load 20 to 40%.
^ Inferred catalyst temp. 950°F (510°C) to 1,300F°(704°C).
^ Number of front O2 switches: 30.

2. The DTCs associated with this test are DTC P0420 (Bank 1 or Y-pipe system) and P0430 (Bank 2). Because an Exponentially Weighted Moving Average algorithm is used for malfunction determination, up to six driving cycles may be required to illuminate the MIL during normal customer driving. If KAM is reset or the battery is disconnected, a malfunction will illuminate the MIL in 2 drive cycles.

General Catalyst Monitor Operation
Monitor execution is once per drive cycle. Typical monitor duration is 700 seconds. In order for the catalyst monitor to run, the HO2S monitor must be complete and Secondary AIR and EVAP system functional with no stored DTCs. If the catalyst monitor does not complete during a particular driving cycle, the already accumulated switch/signal data is retained in Keep Alive Memory and is used during the next driving cycle to allow the catalyst monitor a better opportunity to complete.

Rear HOS2 sensors 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 sensor is used along with the front, fuel control HO2S sensor for each bank. Two sensors are used on an in-line engine; four 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 sensor along with the two front, fuel-control HO2S sensors. Y-pipe system uses three sensors in all. For Y-piped systems, the two front HO2S sensor signals are combined by the PCM software to infer what the HO2S signal would have been in front of the monitored catalyst. The inferred front HO2S signal and the actual single, rear HO2S signal is then used to calculate the switch ratio.

Most vehicles that are part of the Low Emission Vehicle (LEV) catalyst monitor phase-in will monitor less than 100% of the catalyst volume. Often this is the first catalyst brick of the 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 the rear HO2S sensor after the first light-off catalyst can or, after the second catalyst can in a three-can per bank system. (A few application placed the HO2S in the middle of the catalyst can, between the first and second bricks).

Some 2003 model year Partial Zero Emission Vehicles (PZEV) will utilize three sets of HO2S sensors. 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, refer to the Heated Oxygen Sensor (HO2S) Monitor.

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