I have 2 codes PO135 and the other code is for the bank 2 sensor 2 (I can not recall the code). PO135 is for bank 1 sensor 1. The PO135 just started yesterday, the other has been on for about 2 months. My question is would it be wise to replace all 4 O2 sensors, bank 1 and 2 and sensors 1 and 2, or is this one of those if it ain't broke don't fix it kind of things? I know that this is a very talked about topic, and I did a search and filtered through the first couple of pages and could not find anything about replacing all 4 at once.