Find a new mechanic. These are among the most common codes on Contours seen here.
The P0420 is not for a "plugged" pre-cat, it indicates that the pre-cat is not doing its job. Big difference.
Were both exhaust manifolds replaced or only one? If only one, which one and was the original code a p0420 or was it a p0430? Perhaps it was a 430 and the dealer only replaced one pre-cat, now the other has failed (speculation).
Stalling under hard acceleration frequently is due to fuel delivery problems, weak spark, or a dirty MAF.
The P1151 is a weak front upper O2 sensor. Replace it for this code.
The P0420 is for Bank 1 (firewall side). You can putz around with all the other stuff if you want to, but only a new exhaust manifold will properly fix it.
Any exhaust shop can check for clogged cats. Heck, you can buy the guage off of e-Bay for $20 and check it yourself.
If you go with either gutted pre-cats or headers that don't include cats, you will need to get a set of MIL eliminators to keep the CEL from coming on with 420 and 430 codes.
Low idle and stalling are more likely related to the IAC.
Steve
