The converters should not be going on a 3 or 4 year old car. An O2 sensor maybe. Not to mention, the converter is covered by a different warranty, the emissions warranty.
You can get the codes that are stored in OBDII read at most AutoZones, or you can buy a code reader for yourself (I have the ScanGuageII). Getting the codes read will tell you what areas are reporting issues. That is the best place to start.
Once you have the codes, post 'em here and the Answers collective will try to help you out.