As catalysts age their efficiency declines. Catalyst aging is a complex phenomenom, but time at high temperature and contamination such as combustion byproducts from some motor oil additives will slowly reduce their efficiency. As a rule, aged catalysts require higher temperature to achieve a given level of potential catalytic reactions, and catalyst temperature at the beginning of the emission test varies based on "conditioning" - i.e was the car tested immediately after coming in from a spirited drive? Did it idle extensively before the test? Was it shut down while waiting?

Anything you can do to increase catayst temperature on an aged catalyst will enhance catalytic reaction. Drive the car briskly to fully warm up the catalyst and have the test run at an establishement were there is minimum waiting time. NEVER just leave your car somewhere and pick it up later. An few hours "air bath" followed by a startup and a few miniutes of idling before the test starts is recipe for failure! While waiting, keep engine revs at 1500-2000 and load the engine up by turning on the A/C or Defrost (which will engage the A/C compressor) , and NEVER shut it down before the test.

On a modern O2 sensor car, catalyst efficiency can be determined by the amount of O2 in the exhaust (assuming the Lamda system is functioning properly). If it's zero then essentially all potential oxidation reactions are occuring. Any measureable O2 in the exhaust means that not all oxidation reactions are being catalyzed, and a hotter catalyst will usually consume more of the available O2, which will reduce HC and CO.

High temperatures are required for the catalyst to perform, but aging accelerates at high temperature, so it's a double edge sword. Catalyst life varies greatly with catalyst technology and operating conditions, but most will last in the 100-150K range before they are so degraded that they cannot pass field emission test limits.

Does your current and prvious test reports include exhaust O2 content?

Duke