I tried a single defouler, didn't work. I run an emulator now. Had one of those go bad after just over a year. Got another and have been using it probably 20K miles now. I don't think you will experience any difference in how it runs if the code has set. Somewhere I read that the computer only checks the rear 02 sensor on "every third long trip", which meant every third time you drive for more than half an hour. So even if you chop off a cat today, it may not set the CEL for weeks depending on how you drive.