You shouldn't fail as long as the cel doesn't come on while you're there. No code will exist in the car's ecu until the cel comes on. Their computers detect the cel just like the little scanners do. I actually cleared someone's cel the day before a smog test, after they replaced an o2 sensor because it still hadn't turned off after driving about 50 mi. So I just sped up the process and he passed with flying colors. The best thing you can do is to go to a place that does "pass or free retest" in case something happens. If you can get the cat like you said though, you could avoid having to worry about all this.

lol

 

Well I reset the trip odometer after the code was cleared got 45 miles and it popped on again so im gonna go cry myself to sleep I might just give in and go get it fixed somewhere