Hello I'm in need of some input I have 2 codes popping up they come and go periodically. The codes are P0420 and P0171 I bought the 2004 Acura MDX about 2 and a half months ago and check engine light came on twice with those 2 codes but the check engine light would go away the same day or the following day. I bought the O2 sensor but I'm confused as for witch O2 sensor it is due to the fact that the 04 MDX has 3 catalytic converters and 4 sensors. AutoZone did a check engine for me with there machine and on the paper it says Bank 1 cylinder 1 but every where I've checked they never speak of cylinders there located by banks and sensor location so I'm wondering if that cylinder 1 actually means sensor 1? Please help I'm at a lost. I'll post a picture of what I've been going off of. Thanks in advance.

 

My Chilton's manual for 2001-2007 indicates:
P0420= Catalytic efficiency below threshold(REAR CYLINDER HEAD)
P0171= Fuel system too lean (REAR CYLINDER HEAD)

That would cover cylinders 1, 2 and 3 (closest to the firewall), so the upstream O2 sensor on that side should be the one that should get your attention.