Lean Condition

I wouldn't jump to the conclusion that your car is running lean, more likely is that you smell unconverted emissions from your exhaust. I have dumps with a catted h on my Mach and I still smell it sometimes. If you find a solution to stop the smell from entering the cabin, please share.

As far as your check engine light:
First, have you tried disconnecting the battery?
Have you pulled the code to make sure that the porblem you have is the MIL eliminators? My CEL came on one time in my Mach even though I have cats: the problem was actually that the AC waste line was dripping onto the forward passenger O2 sensor, causing it to short. I let the thing dry and shielded it and haven't had the light come on since.

If your car seriously feels weak, get a tune.

Sounds like you've got an exhaust leak somewhere. Weird codes like that are common with exhaust leaks.. the smell is a dead giveaway, especially if you can still smell it at speed. At idle, in reverse, etc, it's normal to smell that crap if you don't have cats.

i agree with the above post. check for exhaust leaks, then look for vacuum leaks and the maf ans such for the lean codes if you don't have a exhaust leak.

BTW 02 sensors NEVER FIX po171 and po 174 lean codes. faulty 02 sensors if they throw a code will throw heater circuit malf. or slow response.

don't listen to autozone diags, 99 percent of them know less then my 70 year old mom, and she don't even drive and could not tell a alternator from a spark plug.