Catalyst Monitoring

Requirements
Regulations require the on-board diagnostic system to monitor the catalyst once per trip. The catalyst is considered to be malfunctioning when hydrocarbon (HO) output exceeds 1.5 times the FTP standards.

Review of Catalyst Function
Three-way catalytic converters contain one or more precious metals (palladium, platinum & rhodium) used to catalyze the unburned hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) which result from the combustion of gasoline. When the catalyst is working properly, the unburned hydrocarbon is oxidized by combining with oxygen, forming water vapor (H20). Carbon monoxide is also oxidized to form carbon dioxide (CO2). Oxides of nitrogen are reduced to nitrogen and oxygen.

To help in these processes, most three-way catalysts also contain a base metal known as cerium. Cerium has the ability to absorb and release excess oxygen in the exhaust stream. This stabilizes the operation of the catalyst and enhances the effectiveness of the precious metals in converting undesirable combustion byproducts to harmless gases. As a catalyst becomes less efficient due to factors such as aging, poisoning or overheating, it's capacity to store and release oxygen generally degrades as well. The OBD II catalyst monitoring diagnostic is based on this correlation between conversion efficiency and oxygen storage capacity.





Theory of Diagnostic Operation
One of the few new components visible on an OBD II SAAB is a "post-catalyst" oxygen sensor mounted downstream of the catalyst. In addition to it's primary role of monitoring catalyst efficiency, the output from the post-catalyst sensor also helps tine tune the fuel mixture in much the same manner as the front sensor always has. The ECM is able to calculate catalyst efficiency by comparing the output from the pre-catalyst sensor with the signal from this new, post-catalyst sensor. This difference in these two readings will result from a characteristic of catalysts known as oxygen storage capacity.

Once the enabling criteria for the catalyst monitoring diagnostic have been met, the ECM compares the frequency and amplitude of the voltage from the pre & post-catalyst sensors. The catalyst monitoring diagnostic procedure takes place once per trip and takes a minimum of 60 seconds to complete.