My check engine light came on yesterday, when it came on it beeped at me i thought it was serious because i have never had one beep at me before, ...

My check engine light came on yesterday, when it came on it beeped at me i thought it was serious because i have never had one beep at me before, so i took it to my machanic and he pulled the code for me. Catalyst System Efficiency Below Threshold (Bank 1)
Now searching this online it seems no to be very serious. It sounds like i could have a exhaust leak or the cat is starting to fail. Has anyone else had this problem? If so what did you do? Should i replace my cat with a high flow? Any inpuut is much apreciated

well if you have an exhaust leak, its possible that it threw that code, but most likely your cats are starting to clog up and fail, i dont know if you think its serious, but this kind of repair can set you back a few hundred dollars.

Same code just popped up... Could it just be a bad 02 sensor?

I had the same thing just the other day on an Explorer I was working on. It wound up being the Catalytic converter was clogged up. They had the code for a few weeks until it clogged all the way up and had to be towed. I put new cat's on it and it was fine. The origional problem was a broken valve spring that was throwing raw fuel out the exhaust and killed the cat. Usually when a cat goes bad it's because something killed it. You may want to make sure you don't have anything else wrong too.

Check this out: http://www.obd-codes.com/p0420

Thanks for heads up... Money tight I was hoping for just a bad02 sensor... But it looks to be the cat... I saw autozone has cats from 600-150 bucks

It could be a bad sensor. Read the link I put up, it does a pretty good job of explaining what all it could be. I knew the one I was working on was the cat's because I knew that it had dumped a lot of raw fuel into the exhaust. Just because the one I worked on had bad cat's doesnt mean yours does.