Exhaust Emission System Diagnostics (Emission control - V8)

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: l Lambda ratio outside of operating band l Lambda heater diagnostic l Lambda period diagnostic l Post-catalytic converter lambda adaptation diagnostic (NAS only) l 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.