I've got a 2003 WRX with 92K miles and a 100% stock exhaust system. I had my vehicle diagnosed by a shop after it deadlined on the highway recently. The shop told me my MAF failed and replaced it. After getting the car back I noticed that the vehicle sounds louder at idle, but it is not idling high. After driving about 10-15 miles I got the Cat Converter efficiency code (P0420). I cleared it only to have it return 50 miles later.
Any suggestions on how to approach this? I've performed a visual inspection of the exhaust system and from the external view nothing appears to be incorrectly installed/missing. Do I need to pull the MAF out and inspect it?