check engine light

[FiremanJay]

well i have a flowmaster exhaust on my 03 h2 and now the check engine light comes on. My mechanic tells me after checking it with the scanalizer that it is a catalitic converter issue. he reset it then 2 days later it came back on. the first time this happened it was the gas cap needed to be replaced of which i did. is there something i need to do for these sensors that are before and after the catalitic converter? im am looking for the K&N air filter will that help? what needs to be done? here in VA we have to get an emissions test every 2 years. is this going to be an ongoing problem?
any help will be greatly appreciated
Thanks
Jay