Secondary Air System Monitoring
5 SECONDARY AIR SYSTEM MONITORINGAfter a cold start condition (e.g. 5 - 40 °C) the air system blows for a certain time air into the exhaust manifold. The exhaust gases will be enriched with oxygen and a post-combustion of HC and CO occurs. By this exothermic reaction the exhaust system will be heated and the time to reach the light-off temperature of the catalyst will be accelerated.
5.1 Pressure-based monitoring of the SAIR system
5.2 General description
The secondary air system comprises a secondary air pump which forces pressurized air into the cylinder head exhaust outlet ports, and a secondary air valve (controlled or self-opening) which prevents the exhaust gas from repulsing, when the pump is inactive. Moreover, it comprises of bores in the cylinder head and hoses or tubes connecting both the pump to the valve, and the valve to the cylinder head.
For the secondary air monitoring, the pressure is generally measured between the pump and the valve, and it is then compared to diagnostic threshold values stored in the ECM. In the secondary air pipe, the pressure sensor should be installed upstream of the valve, and as close as possible to it or in it.
Possible fault types of this system are blockages or leakages of the air supply, a reduced pump performance or a stuck open valve.
When the secondary air injection is active, the pressure in the pipes downstream of the secondary air pump increases. The difference between this secondary air pressure and the ambient air pressure is used as a measure for the secondary air mass added to the exhaust gas (see Figure below). In case the detected pressure rise falls below the normal pressure of the PASS system by a certain value, OBD relevant leakages are present, or the overall performance of the system is reduced. A system blockage results in a pressure rise.
The normal pressure of the system depends on various factors like the temperature, the pump voltage, the altitude and the exhaust gas back pressure.
If the valve is controlled externally, the catalyst heating is followed by a valve leakage check, during which the pump works against the closed valve. The pressure increase gives information on the existence of a fault. In reality, the pressure signal is highly noisy (due to the exhaust gas pulsations), and therefore a low pass filter has to be applied to it (see Figure below). The decision whether a fault is existent or not is then taken by comparing the filtered value with a modeled setpoint value.
Additionally, the amplitude of the pressure signal can also be evaluated, if necessary.
To guarantee a reliable bank-specific fault allocation for the two-bank system with a secondary air pump, two separately controlled secondary air valves are necessary (electrically controlled or vacuum controlled). Fault types are distinguished by activating the valves alternately (see following Figure).
The above table gives an overview of the different phases. Phases 21 and 22 are only applicable for a two-bank system, and phase 2 is optional for externally activated valves.
For the arrangement of the pressure sensors for a secondary air pump that serves two cylinder banks, see Figure below.
The pressure sensor is installed upstream of the split for the two banks, and thus the overall pressure in the secondary air system is measured.
5.2.1 Pressure Sensor Monitoring
5.2.1.1 Signal Range Fault: Voltage Signal exceeds upper threshold, P2433/P2438
Monitoring Strategy
The voltage at the secondary air sensor must not exceed an upper threshold.
Malfunction Criteria
The voltage signal of the pressure sensor is monitored and must not exceed an upper threshold. When the signal exceeds the upper threshold, a fault is detected.
Flowchart
5.2.1.2 Signal Range Fault: Voltage Signal falls below lower threshold, P2432/P2437
Monitoring Strategy
The voltage at the secondary air sensor must not fall below a lower threshold.
Malfunction Criteria
The voltage signal of the pressure sensor is monitored and must not fall below a lower threshold. When the signal falls below the lower threshold, a fault is detected.
Flowchart
5.2.2 General Enable Conditions (for all the following monitors)
Physical Enable Conditions
- Ambient air pressure is valid
- Altitude > minimum height factor x 1013 hPa
- Engine temperature is within thresholds
- Intake air temperature is within thresholds
- Main catalyst, no primary catalyst, Mono-Lambda Control:
o Catalyst temperature is lower than threshold value
- Stereo-Lambda Control:
o Maximum catalyst temperature of bank 1 and bank 2 is lower than a threshold value
- Engine air mass is within thresholds
- Charge variation per combustion cycle is within thresholds
5.2.3 Rationality Fault, P2431/P2436 - Phase 0
Additional Enable Conditions for this Diagnostic:
- Ambient air pressure signal is valid
- No restart
- Secondary air injection has ended
Monitoring Strategy
Post to the active secondary air injection phase, the difference between the secondary air pressure and the ambient air pressure must not exceed an upper threshold or fall below a lower threshold.
Malfunction Criteria
During the initialization phase of the ECM, the measured absolute pressure in the secondary air pipe at engine-off is stored. Simultaneously, a rationality check of the pressure sensor signal is performed. For this check, the absolute pressure signal from the secondary air sensor is compared to the ambient air pressure sensor in the ECM. If the difference between the secondary air pressure and the ambient air pressure falls below a lower threshold or exceeds an upper threshold, a defective secondary air pressure sensor is detected.
Flowchart
5.2.4 Secondary Mass Air Flow Monitoring
5.2.4.1 Secondary Mass Air Flow below lower threshold, P0491/P0492 - Phase 1
Monitoring Strategy
The ratio of the measured secondary air pressure and the modeled secondary air pressure must neither exceed an upper threshold value nor fall below a lower threshold value.
Malfunction Criteria for a one-bank system
During the secondary air injection, which controls the secondary air pump and the secondary air valve, it is measured if enough secondary air mass is injected into the exhaust system. The ratio of the measured secondary air pressure and the modeled secondary air pressure is compared to a diagnostic threshold value. In the case of a blockage in the secondary air system, the pressure ratio exceeds an upper threshold value. If the pressure ratio falls below a lower threshold, a leakage in the secondary air system is detected.
In the case of a system with one secondary air pump, one pressure sensor and one secondary air valve, an evaluation of the pressure pulsations may additionally be performed, in order to detect an insufficient secondary air mass caused by a leakage in the secondary air supply pipe during the measurement phase. For that purpose, the absolute value of the area between the currently measured pressure value and the filtered pressure value is integrated and then divided by the measurement period. If the mean pressure difference falls below the expected value, which depends on the engine operating point, a leakage is detected.
Malfunction Criteria for a two-bank system - Phase 21 and Phase 22
For a bank specific fault allocation, a system consisting of a secondary air pump, a pressure sensor and two secondary air valves requires an alternating activation of the secondary air valves. For this purpose, the secondary air valves are alternately activated when the secondary air pump is active, and the relative secondary air ratio is determined for each bank and then compared to the threshold values. If an air ratio falls below a lower threshold value, a leakage is detected, and if an air ratio exceeds an upper threshold value, a blockage in the secondary air system is detected.
As a large pressure drop may occur at the secondary air valve, leakage detection by means of the pressure ratio can be very difficult, depending on the respective secondary air valve and the secondary air supply. For this reason, an additional means for leakage detection, the evaluation of pressure pulsations, must be implemented. For that purpose, the absolute value of the area between the currently measured pressure value and the filtered pressure value is integrated and then divided by the measurement period. If the mean pressure difference falls below the expected value, which depends on the engine operating point, a fault is detected.
If, additionally to the detection of a fault during the bank specific phase, NO fault is detected during the valve check in phase 2 or phase 3, insufficient secondary air is detected for the respective bank.
Flowchart
Flowchart
5.2.4.2 Secondary Air Valve stuck open, P2440/P2442
During Phase 2
Monitoring Strategy
The ratio of the measured secondary air pressure and the modeled secondary air pressure must not fall below a lower threshold value.
Malfunction Criteria
Following the secondary air injection into the exhaust system, the secondary air valve(s) is/are externally closed. The secondary air pump, however, remains activated, and the secondary air system between the secondary air pump and the secondary air valve(s) is monitored.
To assess whether a stuck open valve fault is present, the measured secondary air pressure is compared to the expected secondary air pressure. The ratio resulting from this comparison is compared to a threshold value. A valve leakage is present, if the ratio falls below the threshold value.
In case of a two bank system (with one secondary air pump), additionally to the fault during phase 2, a fault must have occurred in the bank specific phase 21 or 22. In this case, a stuck open valve is detected on the respective opposite bank, which means:
Fault during phase 21 AND fault during phase 2 AND NO fault during phase 1: Stuck open valve on bank 2
Fault during phase 22 AND fault during phase 2 AND NO fault during phase 1: Stuck open valve on bank 1
Flowchart
During Phase 3
Monitoring Strategy
The mean value of the absolute difference of the measured pressure and the initialization pressure must not exceed an upper threshold, its maximum deviation must not exceed an upper threshold and its minimum deviation must fall below a lower threshold.
Malfunction Criteria
To detect a stuck open valve during the offset phase, the difference between the measured pressure value and the filtered value of the pressure sensor, the maximum, and the minimum are determined. In case the determined maximum exceeds an upper threshold value, the minimum falls below a lower threshold value, and the mean value of the deviation exceeds a threshold value, a stuck open valve is detected. The threshold values depend on the engine's current operating point.
In the case of a two bank system (with one secondary air pump), additionally to the fault during phase 3, a fault must have occurred in the bank specific phase 21 or 22. In this case, a stuck open valve is detected on the respective opposite other bank, which means:
Fault during phase 21 AND fault during phase 3 AND NO fault during phase 1: Stuck open valve on bank 2
Fault during phase 22 AND fault during phase 3 AND NO fault during phase 1: Stuck open valve on bank 1
Flowchart
5.2.4.3 Secondary Air System Malfunction, P0410/P2446 - Phase 3
Monitoring Strategy
The secondary air pressure must not exceed the setpoint pressure value for a defined time period.
Malfunction Criteria
After the secondary air system leakage monitoring or the bank specific monitoring, respectively, the secondary air pump and the secondary air valve(s) is/are no longer activated. In this phase, the secondary air pressure should return to the initialization pressure. A secondary air system malfunction is detected, when the pressure difference value between these two pressures exceeds the threshold value for a calibratable time period. The malfunction can be caused by a secondary air pump that continues to run (in case of self-opening valves), or by a defective secondary air pressure sensor.
Flowchart
5.3 In-Use Monitoring Performance Ratio
Incrementing the numerator
The secondary air monitor is a symmetrical monitor; the monitoring procedure in the case of a faulty system is the same as that for proper function. The numerator is incremented by one when the applicable cycle flags are set. If the secondary air run-time is very low (e.g. due to drive away under high load condition), the secondary air valve monitor is not completed and the numerator is not incremented.
Incrementing the denominator
The denominator is incremented by one when the conditions for incrementing the general denominator, in accordance with CCR (d) (4.3.2.) (E) (ii), are fulfilled, and when the secondary air injection has been active for a minimum of 10 seconds.