Catalyst Efficiency Monitor
The Catalyst Efficiency Monitor uses an oxygen sensor after the catalyst to infer the hydrocarbon efficiency based on oxygen storage capacity of the ceria and precious metals in the washcoat. Under normal, closed-loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear HO2S very slow and reduces the amplitude of those. As catalyst efficiency deteriorates due to thermal and/or chemical deterioration, its ability to store oxygen declines and the post-catalyst HO2S signal begins to switch more rapidly with increasing amplitude. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposition on the front brick of the catalyst), not thermal deterioration.
Index Ratio Method Using a Switching HO2S Sensor
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 (air mass region), the total signal length of the rear HO2S is divided by the total signal length of front HO2S to compute a catalyst index ratio. An index ratio near 0.0 indicates high oxygen storage capacity, hence high HC efficiency. An index 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.
If the catalyst monitor does not complete during a particular driving cycle, the already-accumulated switch/signal length 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, even under short or transient driving conditions.
If the catalyst monitor runs to completion during a driving cycle, it will be allowed to run again and collect another set of data during the same driving cycle. This would allow the catalyst monitor to complete up to a maximum of two times per driving cycle, however, the in-use performance ratio numerator for the catalyst monitor will only be allowed to increment once per driving cycle. For example, if the catalyst monitor completes twice during the current driving cycle, the catalyst monitor in-use performance numerator will be incremented once during the current driving cycle and will incremented again for the second completion on the following driving cycle, after the catalyst monitor entry condition have been met.
Index Ratio Method Using a Wide Range HO2S Sensor (UEGO)
The switching HO2S control system compares the HO2S signals before and after the catalyst to assess catalyst oxygen storage. The front HO2S signal from UEGO control system is used to control to a target A/F ratio and does not have "switches" As a result, a new method of catalyst monitor is utilized.
The UEGO catalyst monitor is an active/intrusive monitor. The monitor performs a calibratable 10-20 second test during steady state rpm, load and engine air mass operating conditions at normal vehicle speeds. During the test, the fuel control system remains in closed loop, UEGO control with fixed system gains. In order to assess catalyst oxygen storage, the UEGO catalyst monitor is enabled during part-throttle, closed-loop fuel conditions after the engine is warmed-up and inferred catalyst temperature is within limits. While the catalyst monitoring entry conditions are being met, the rear HO2S signal length is continually being calculated. When the required total calibrated time has been accumulated, the total voltage signal length of the rear HO2S is divided by a calibrated threshold rear HO2S signal length to compute a catalyst index ratio. The threshold rear HO2S signal is calibrated as a function of air mass using a with a catalyst with no precious metal. This catalyst defines the worst case signal length because it has no oxygen storage. If the monitored catalyst has sufficient oxygen storage, little activity is observed on the rear HO2S voltage signal. An index ratio near 0.0 indicates high oxygen storage capacity, hence high HC/NOx efficiency. As catalyst oxygen storage degrades, the rear HO2S voltage signal activity increases. An index ratio near, 1.0 indicates low oxygen storage capacity, hence low HC/NOx efficiency. If the actual index ratio exceeds the calibrated threshold ratio, the catalyst is considered failed.
Integrated Air/Fuel Method
The Integrated Air/Fuel Catalyst Monitor assesses the oxygen storage capacity of a catalyst after a fuel cut event. The monitor integrates how much excess fuel is needed to drive the monitored catalyst to a rich condition starting from an oxygen-saturated, lean condition. Therefore, the monitor is a measure of how much fuel is required to force catalyst breakthrough from lean to rich. To accomplish this, the monitor runs during fuel reactivation following a Decel Fuel Shut Off (DFSO) event. The monitor completes after a calibrated number of DFSO monitoring events have occurred. The IAF catalyst monitor can be used with either a wide range O2 sensor (UEGO) or a conventional switching sensor (HEGO).
The monitor runs during reactivation fueling following an injector cut. The diagram below shows examples of one DFSO event with a threshold catalyst and with a Full Useful Life catalyst where:
- INJON = # of injectors on.
- CMS is the catalyst monitor sensor voltage. When the rear O2 sensor crosses 0.45 volts (i.e. rich) the monitor will complete for the given DFSO event.
- LAM (LAMBDA) is the front O2 sensor (UEGO) signal.
- CATMN_IAF_SUM is the integral from the equations above (Y axis on the right).
In this example, CATMN_IAF_SUM is small because it doesn't take much fuel to break though a low oxygen storage threshold catalyst.
In this example, CATMN_IAF_SUM is much larger because it takes a substantial amount of fuel to break though a high oxygen storage threshold catalyst.
There are two sets of entry conditions into the IAF catalyst monitor. The high level entry conditions determine that the monitor would like to run following the next injector fuel cut event. The lower level entry conditions determine that the fuel cut-off event was suitable for monitoring and the monitor will run as soon as the injectors come back on.
- The high level entry conditions are met when:
- There are no senor/hardware faults
- The base monitor entry conditions have been met (ECT, IAT, cat temp, fuel level, air mass)
- Required number of DFSO monitoring event have not yet completed
- The lower level entry conditions are met when:
- The injectors are off
- The catalyst is believed to be saturated with oxygen (rear O2 indicates lean)
- The catalyst/rear O2 has been rich at least once since the last monitor event.
General Catalyst Monitor Operation
Rear HO2S sensors can be located in various ways to monitor different kinds of exhaust systems. In-line engines and many V-engines are monitored by 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 use three sensors in all. For Y-pipe systems which utilize switching front O2 sensors, the two front HO2S sensor signals are combined by the 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.
Many vehicles monitor less than 100% of the catalyst volume - often the first catalyst brick of the catalyst system. Partial volume monitoring is done on LEV-II vehicles in order to meet the 1.75 * emission-standard threshold for NMHC and NOx. The rationale for this practice is that the catalysts nearest the engine deteriorate first, allowing the catalyst monitor to be more sensitive and illuminate the MIL properly at lower emission standards.
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 applications placed the HO2S in the middle of the catalyst can, between the first and second bricks.)
The new Integrated Air/Fuel Catalyst Monitor can be used to monitor the entire catalyst volume, even on LEV-II vehicles.
Index ratios for ethanol (Flex fuel) vehicles vary based on the changing concentration of alcohol in the fuel. The malfunction threshold typically increases as the percent 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 % alcohol in the fuel.
(Normal gasoline is allowed to contain up to 10% ethanol (E10)).
Vehicles with the Index Ratio Method Using a Switching HO2S Sensor employ an Exponentially Weighted Moving Average (EWMA) algorithm to improve the robustness of the catalyst monitor. During normal customer driving, a malfunction will illuminate the MIL, on average, in 3 to 6 driving cycles. If KAM is reset (battery disconnected) or DTCs are cleared, a malfunction will illuminate the MIL in 2 driving cycles. See the section on EWMA for additional information.
Vehicles with the Index Ratio Method Using a Wide Range HO2S Sensor (UEGO) or the Integrated Air/Fuel catalyst monitor employ an improved version of the EWMA algorithm.
- The EWMA logic incorporates several important CARB requirements. These are:
- Fast Initial Response (FIR): The first 4 tests after a battery disconnect or code clear will process unfiltered data to quickly indicate a fault. The FIR will use a 2-trip MIL. This will help the service technician determine that a fault has been fixed.
- Step-change Logic (SCL): The logic will detect an abrupt change from a no-fault condition to a fault condition. The SCL will be active after the 4th catalyst monitor cycle and will also use a 2-trip MIL. This will illuminate the MIL when a fault is instantaneously induced.
- Normal EWMA (NORM): This is the normal mode of operation and uses an Exponentially Weighted Moving Average (EWMA) to filter the catalyst monitor test data. It is employed after the 4th catalyst test and will illuminate a MIL during the drive cycle where the EWMA value exceeds the fault threshold. (1 trip MIL).
Starting in the 2010 1/2 Model Year and later, the catalyst monitor will employ catalyst break-in logic. This logic will prevent the catalyst monitor from running until after a catalyst break-in period.
The catalyst monitor will not run on a new vehicle from the assembly plant until 60 minutes of time above a catalyst temperature (typically 800 to 1100 deg F) has been accumulated or 300 miles has elapsed.
New modules at the assembly plant will have an NVRAM flag initialized to delay the catalyst monitor. Service modules and re-flash software will have the flag set to allow that catalyst monitor to run. The flag cannot be reset to delay the catalyst monitor from running by any tool or service procedure.
| DTCs | P0420 Bank 1 (or Y-pipe), P0430 Bank 2 |
| Monitor execution | once per driving cycle |
| Monitor Sequence | HO2S response test complete and no DTCs (P0133/P0153) prior to calculating switch ratio, no SAIR pump stuck on DTCs (P0412/P1414), no evap leak check DTCs (P0442/P0456), no EGR stuck open DTCs (P0402) |
| Sensors OK | ECT, IAT, TP, VSS, CKP, MAF, no misfire DTCs (P0300, P0310), no ignition coil DTCs (P0351-P0358), no fuel monitor DTCs (P0171, P0172, P0174. P0175), no VCT DTCs (P0010-P0017, P052A, P052B, P0344, P0365, P0369- bank 1) (P0018 thru P0025, P052C, P052D, P0349, P0390, P0394- bank 2). no evap system DTCs (P0443, P0446, P0455, P0457, P1450), no ETC system DTCs (P0122, P0123, P0222, P0223, P02135) (P2101, P2107, P2111, P2112) (P0600, P060A, P060B, P060C, P061B, P061C, P061D, P1674, U0300). |
| Monitoring Duration | Approximately 700 seconds during appropriate FTP conditions (approximately 100 to 200 oxygen sensor switches are collected) for switching O2 control sensors Approximately 10 to 20 seconds for wide range O2 index ratio monitor. 3 Decel Fuel Cutoff events for IAF catalyst monitor |
| Entry Condition | Minimum | Maximum |
| Time since engine start-up (70 °F start) | 330 seconds | |
| Engine Coolant Temp | 170 °F | 230 °F |
| Intake Air Temp | 20 °F | 180 °F |
| Time since entering closed loop fuel | 30 seconds | |
| Inferred Rear HO2S sensor Temperature | 900 °F | |
| EGR flow (Note: an EGR fault disables EGR) | 1% | 12% |
| Throttle Position | Part Throttle | Part Throttle |
| Rate of Change of Throttle Position | 0.2 volts / 0.050 s | |
| Vehicle Speed | 5 mph | 70 mph |
| Fuel Level | 15% | |
| First Air Mass Cell | 1.0 lb/min | 2.0 lb/min |
| Engine RPM for first air mass cell | 1, 000 rpm | 1, 300 rpm |
| Engine Load for first air mass cell | 15% | 35% |
| Monitored catalyst mid-bed temp. (inferred) for first air mass cell | 850 °F | 1, 200 °F |
| Second Air Mass Cell | 2.0 lb/min | 3.0 lb/min |
| Engine RPM for second air mass cell | 1, 200 rpm | 1, 500 rpm |
| Engine Load for second air mass cell | 20% | 35% |
| Monitored catalyst mid-bed temp. (inferred) for second air mass cell | 900 °F | 1, 250 °F |
| Number of front O2 switches required for second air mass cell | 70 | |
| Third Air Mass Cell | 3.0 lb/min | 4.0 lb/min |
| Engine RPM for third air mass cell | 1, 300 rpm | 1, 600 rpm |
| Engine Load for third air mass cell | 20% | 40% |
| Monitored catalyst mid-bed temp. (inferred) for third air mass cell | 950 °F | 1, 300 °F |
| Number of front O2 switches required for third air mass cell | 30 | |
|
NOTE:
(Engine rpm and load values for each air mass cell can vary as a function of the power-to-weight ratio of the engine, transmission and axle gearing and tire size.)
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| Entry Condition | Minimum | Maximum |
| Time since engine start-up (70 °F start) | 330 seconds | |
| Engine Coolant Temp | 170 °F | 230 °F |
| Intake Air Temp | 20 °F | 180 °F |
| Time since entering closed loop fuel | 30 sec | |
| Inferred Rear HO2S sensor Temperature | 900 °F | |
| EGR flow (Note: an EGR fault disables EGR) | 1% | 12% |
| Throttle Position | Part Throttle | Part Throttle |
| Rate of Change of Throttle Position | 0.2 volts / 0.050 s | |
| Vehicle Speed | 20 mph | 80 mph |
| Fuel Level | 15% | |
| Air Mass | 2.0 lb/min | 5.0 lb/min |
| Engine RPM | 1, 000 rpm | 2, 000 rpm |
| Engine Load | 20% | 60% |
| Monitored catalyst mid-bed temp. (inferred) for first air mass cell | 850 °F | 1, 200 °F |
|
NOTE:
(Engine rpm, load and air mass values can vary as a function of the power-to-weight ratio of the engine, transmission and axle gearing and tire size.)
|
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| Entry Condition | Minimum | Maximum |
| Engine Coolant Temp | 160 °F | 250 °F |
| Intake Air Temp | 20 °F | 140 °F |
| Inferred catalyst mid-bed temperature | 900 °F | 1500 °F |
| Fuel Level | 15% | |
| Air Mass | 2.0 lb/min | |
| Minimum inferred rear O2 sensor temperature | 800 °F | |
| Fuel monitor learned within limits | 97% | 103% |
| Rear O2 sensor rich since last monitor attempt | 0.45 Volts | |
| Rear O2 sensor lean with injectors off (voltage needed to enter monitor) | 0.1 Volts | |
| Rear O2 sensor reads rich after fuel turned back on (voltage needed to complete monitor) | 0.45 Volts |
| Catalyst monitor index ratio > 0.75 (bank monitor) Catalyst monitor index-ratio > 0.60 (Y-pipe monitor) Catalyst monitor index ratio > 0.50 for E10 to > 0.90 for E85 (flex fuel vehicles) |
Mode $06 reporting for IAF Catalyst Monitor
The catalyst monitor results are converted to a ratio for Mode $06 reporting to keep the same look and feel for the service technician. The equation for calculating the Mode $06 monitor result is:
- (Actual reactivation fuel/ Good catalyst reactivation fuel)
Good catalyst reactivation fuel is intended to represent what the monitor would measure for a green catalyst.
| Monitor ID | Test ID | Description | |
| $21 | $80 | Bank 1 index-ratio and max. limit (P0420/P0430) | unitless |
| $22 | $80 | Bank 2 index-ratio and max. limit (P0420/P0430) | unitless |