Have cleaned (4 times) MAF and replaced it. Engine still rough with engine light on and P1131 code. Points to 02 sensor. How do I tell which one? Where is it? I found one on left side. Can't find one on right side. I was told that there are 2 above the cat converter and 2 below - right and left side. I think the ones below the cat converter are a different code. If that's true, then how do I tell if it's the right of left one? Note - I'm a decent backyard mechanic, but don't know to use a voltmeter or do auto electric testing. How hard is it to replace?