Exhaust Emission System Diagnostics
Exhaust Emission System Diagnostics
The engine management ECM contains an on-board diagnostics (OBD) system which performs a number of diagnostic routines for detecting problems associated with the closed loop emission control system. The diagnostic unit monitors ECM commands and system responses and also checks the individual sensor signals for plausibility, these include:
^ Lambda ratio outside of operating band
^ Lambda heater diagnostic
^ Lambda period diagnostic
^ Post-catalytic converter lambda adaptation diagnostic (NAS only)
^ Catalyst monitoring diagnostic
Lambda ratio outside operating band
The system checks to ensure that the system is operating in a defined range around the stoichiometric point. If the system determines that the upper or lower limits for the air:fuel ratio are being exceeded, the error is stored as a fault code in the ECM diagnostic memory (the MIL light is illuminated on NAS vehicles).
Lambda heater diagnostic
The system determines the heater current and supply voltage so that the heater's resistance can be calculated. After the engine has been started, the system waits for the heated oxygen sensors to warm up, then calculates the resistance from the voltage and current measurements. If the value is found to be outside of the upper or lower threshold values, then the fault is processed (the MIL light is illuminated on NAS vehicles).
Lambda period diagnostic
The pre-catalytic converter sensors are monitored. As the sensors age, the rich to lean and the lean to rich switching delays increase, leading to increased emissions if the lambda control becomes inaccurate. If the switching period exceeds a defined limit, the sensor fault is stored in the ECM diagnostic memory (the MIL light is illuminated on NAS vehicles).
Post-catalytic converter lambda adaptation diagnostic (NAS only)
On NAS vehicles the ageing effects of the pre-catalytic converter sensors are compensated for by an adaptive value derived from the post-catalytic converter sensors. This is a long term adaption which only changes slowly. For a rich compensation the additive value is added to the rich delay time. For a lean compensation, the adaptive value is added to the lean delay time. The adaptive time is monitored against a defined limit, and if the limit is exceeded, the fault is stored in the ECM's diagnostic memory and the MIL light is illuminated on the instrument pack.
Catalyst monitoring diagnostic
On NAS specification vehicles the catalysts are monitored both individually and simultaneously for emission pollutant conversion efficiency. The conversion efficiency of a catalyst is monitored by measuring the oxygen storage, since there is a direct relationship between these two factors. The closed loop lambda control fuelling oscillations produce pulses of oxygen upstream of the catalyst, as the catalyst efficiency deteriorates its ability to store oxygen is decreased. The amplitudes of the signals from the pre-catalytic and post-catalytic converter heated oxygen sensors are compared. As the oxygen storage decreases, the post-catalytic converter sensor begins to follow the oscillations of the pre-catalytic converter heated oxygen sensors. Under steady state conditions the amplitude ratio is monitored in different speed/load sites. There are three monitoring areas, and if the amplitude ratio exceeds a threshold in all three areas the catalyst conversion limit is exceeded; the catalyst fault is stored in the diagnostic memory and the MIL light is illuminated on the instrument pack. There is a reduced threshold value for both catalysts monitored as a pair. In either case, a defective catalyst requires replacement of the downpipe assembly.
In the case of a catalytic converter failure the following failure symptoms may be apparent:
^ MIL light on after 2 driving cycles (NAS market only).
^ High exhaust back pressure if catalyst partly melted.
^ Excessive emissions
^ Strong smell of H2S (rotten eggs).
Oxygen sensor voltages can be monitored using 'Testbook', the approximate output voltage from the heated oxygen sensors with a warm engine at idle and with closed loop fuelling active are shown in the table.
Mass Air Flow Sensor and Air Temperature Sensor
The engine management ECM uses the mass air flow sensor to measure the mass of air entering the intake and interprets the data to determine the precise fuel quantity which needs to be injected to maintain the stoichiometric air:fuel ratio for the exhaust catalysts. If the mass air flow sensor fails, lambda control and idle speed control will be affected and the emission levels will not be maintained at the optimum level. If the device should fail and the ECM detects a fault, it invokes a software backup strategy.
The air temperature sensor is used by the engine management ECM to monitor the temperature of the inlet air. If the device fails, catalyst monitoring will be affected. The air temperature sensor in integral to the mass air flow sensor.
Throttle Position Sensor
If the engine management ECM detects a throttle position sensor failure, it may indicate a blocked or restricted air intake filter. Failure symptoms may include:
^ Poor engine running and throttle response
^ Emission control failure
^ No closed loop idle speed control
^ Altitude adaption is incorrect
If a signal failure should occur, a default value is derived using data from the engine load and speed.
Atmospheric pressure will vary with altitude and have a resulting influence on the calculations performed by the ECM in determining the optimum engine operating conditions to minimise emissions. The following are approximate atmospheric pressures for the corresponding altitudes:
^ 0.96 bar at sea level
^ 0.70 bar at 2,750 m (9,000 ft.)