Hi All,

This morning my Check Engine Light came on (with nothing noticeable while driving or idle speed, etc). I finally got over to Checker this evening and pulled the code and got a P0420: Catalyst system efficiency below threshold (Bank 1). From the last snapshot, the STFT and LTFT were both 3.1% at about 1800 rpm. So, after searching this site and others, it appears this code could be from a few causes:

Bad rear 02 sensor
Bad cat or leaks in pipe around cat
Coolant temps

The front 02 sensor was already replaced a couple of years ago. A few months back I got a Magnaflow exhaust and a hi-flow (aftermarket) cat that was welded up by a local shop with a good reputation. Not fully understanding how the OBDC diagnostics work, it seems fishy that the STFT and LTFT were the same, but they also appear reasonable (I found write ups that said to ignore LTFTs below 10%). So, tomorrow morning I'll probably check my coolant and hunt around the exhaust pipe. If anyone has any advice on how best to proceed, it would be greatly appreciated!