no it wont kill it. it'll just run in loop since the O2 will read foul. so it will basically just give bad gas mileage due to a number of reasons I don't want to go into. your alright unless you need to pass emmisions.

 

A P0420 on it's own isn't going to have any effect on fuel mileage, and it's not going to put the engine management back into open loop. The sensors that manage the fuel mixture are upstream of the catalyst, so they will continue to function just fine. The O2 sensor behind the catalyst has absolutely nothing to do with how the engine runs.
The only thing that would affect the engine would be a catalyst that was clogged to the point that it increased back pressure, but this would be very noticeable.

In my experience, aftermarket cats are garbage. I have had more customers that I can remember who didn't pass an emissions test, went out and spent a few hundred bucks on an aftermarket cat, only to replace it a few months later with an expensive factory unit.

 

does it literally effect gas mileage?

 

Yes.

As I understand it, the car will run richer than normal - thus causing your gas mileage to drop.

 

well heres the thing... the light came on and i took a road trip after unhooking the battery to turn off the light. i still got 487 miles for a tank of gas. is that usual or should i be getting more?

 

You just reset the computer, but the problem still remains. Eventually the computer will re-learn the parameters of the engine, and trip the check engine light again.

 

yes the i4... but thats only on highway miles, and its a big tank. does anyone else get 487 a tank?

 

seems about right, I would either replace the cat (if you hear rattling underneath your car). Or replace it with a test pipe and install an o2 simulator.

When my cat converter went bad, I didn't notice any mpg difference.

 

how much money are we talking about ? haha im a poor college student

 

I bought a new obd2 cat off the for sale section for like 50 bucks, and the local exhaust shop charged about 30 to weld it on.

 

very nice, where do i look for one like that?

 

An obd2 scanner can be purchased on ebay, autozone, etc. However your CEL will keep coming back on until you have fixed the problem. Get a new catylitic converter/exhaust. They arent that expensive and your car will thank you. If you take it to a place that specializes in exhaust, mufflers, etc, they will usually give you a better deal then a stealership. Remember if you take of your car it will take care of you.