if you just bought the car and the dealer said they would fix the p0420 let them do it. subaru dealers fix it for customers by selling them new cats. it should be interesting to see what your dealer does for you.
and the 02 extender is not the fix for the p0420. it is the solution when all else fails. it is in every ones best interest to find the actual cause and correct it. lots of time a new front o2 sensor from subaru and maybe rear o2 sensor will do the trick. the list of things that can cause this code is very long and ranges from bad cats (which is really rare) to an exhaust or intake leak.
if the engine does have an ignition ''ping'' or ''knock'' you WILL have a CEL for the knock sensor., p0325. which you may have in addition to the p0420. but you will not know unless you cheack it with a reader. the symptom is a sluggish off the line, no power and lousy MPG. get the codes read again, all of them. you can get good knock sensors from ebay for under $20. if you are in a hurry, make sure it ships from the US, not china.
screeching under load is probably one of your belts, AC or steering pump / alt. as a test you can remove them one at a time and test drive the car for a short drive. you can drive it without the alt belt as long as the battery has enough charge. do not go too far.
does the screech change when you turn the AC on / off??
good luck, and let us know what you learn.
Last edited by johnegg; 12-10-2012 at 07:02 PM..