My roomie has a 2006 Nissan Sentra with a 1.8 that had the CEL light come on the other day and stay on. The code is a P0155. Any suggestions on where to look and what to test, or does that pretty much mean an oxygen sensor replacement? Also, which one of the 2 oxygen sensors would that indicate? I have an actron scanner, an actron sensor tester (I forget what model, but I bought it at Autozone 6 or 7 years ago), and she picked up a Haynes manual for the car the other day. Thanks for any assistance!