If you have anything in your car that is broken, you will fail, and the "guarantee" does not apply. The guarantee only works if everything is working. And if everything in your car is working fine, your car should pass no problem, so that product is shit.
Honestly, take it from a stubborn guy, don't waste anymore time trying to guess, it's not worth the hassle. Most likely your cat or o2 sensor are toast, and BMW can tell you that in under an hour.