Exhaust Emission Control Components - (From 99MY)

Catalytic converters
The catalytic converters are located in each of the front pipes from the exhaust manifolds. The catalytic converter's housings are fabricated from stainless steel and are fully welded at all joints. Each catalytic converter contains two elements of an extruded ceramic substrate which is formed into a honeycomb of small cells with a density of 62 cells/sq.cm. The ceramic element is coated with a special surface treatment called 'washcoat' which increases the surface area of the catalyst element by approximately 7000 times. A coating is applied to the washcoat which contains the precious elements platinum, palladium and rhodium in the following relative concentrations: 1 Pt: 21.6 PD :1 Rh.

The metallic coating of platinum and palladium oxidize the carbon monoxide and hydrocarbons and convert them into water (H20) and carbon dioxide (C02). The coating of rhodium removes the oxygen from nitrogen oxide (NOx) and converts it into nitrogen (N2).


! CAUTION: Catalytic converters contain ceramic material which is very fragile. Avoid heavy impacts on the converter casing.

Heated Oxygen (HO2S) Sensors:

The heated oxygen sensor is an integral part of the exhaust emission control system and is used in conjunction with the catalytic converters and the engine management control ECM to ensure that the air:fuel mixture ratio stays around the stoichiometric ideal, where the catalytic converters are most effective. Combinations of four (NAS only) or two heated oxygen sensors are used in the exhaust system, dependent on market legislation.

The heated oxygen sensors are screwed into threaded mountings welded into the front exhaust pipes at suitable locations. They are used to detect the level of residual oxygen in the exhaust gas to provide an instantaneous indication of whether combustion is complete. By positioning sensors in the stream of exhaust gases from each exhaust manifold, the engine management system is better able to control the fuelling requirements on each bank independently of the other. This facilitates much closer control of the air:fuel ratio and optimizes catalytic converter efficiency.


! CAUTION: HO2S sensors are easily damaged by dropping, excessive heat or contamination care must be taken not to damage the sensor tip or housing.


The sensors consist of a ceramic body (Galvanic cell) which is practically a pure oxygen-ion conductor made from a mixed oxide of zirconium and yttrium. The ceramic is then coated with gas-permeable platinum, which when heated to a sufficiently high temperature (above 350° C) generates a voltage which is proportional to the oxygen content in the exhaust gas stream.

The sensor is protected by an outer tube with a restricted flow opening to prevent the sensor's ceramic from being cooled by low temperature gases at start up. The pre-catalytic sensors are identified by three slots in the protective tube, whereas the post-catalytic sensors have four square indentations and a hole in the end of the protective tube (NAS only).

The HO2S sensors should be treated with extreme care, since the ceramic material within them can be easily cracked if they are dropped, banged or over-torqued. The sensors should be torqued to the recommended values indicated in the repair procedures. Apply anti-seize compound to the sensor's threads when refitting.


! WARNING: Some types of anti-seize compound used in service are a potential health hazard. Avoid skin contact.


! WARNING: To prevent personal injury from a hot exhaust system, do not attempt to disconnect any components until the exhaust system has cooled down.


! CAUTION: Do not allow anti-seize compound to come into contact with the tip of the sensor or enter the exhaust system.


! NOTE: A new HO2S sensor is supplied with pre-treated anti-seize compound.