I just want to start by saying thanks for all the information on this site. It is incredible what you can find when you search. I am a very new 95 4runner owner after recently selling my 4th FJ60! So far I have replaced the coils in the rear, put my 32 BFG ATs on the stock rims (a little tight), Yakima Racks, Clarion stereo with amp, and hella reverse lights on. I will post some pics today. Now, on to the fun. My CEL came on so I jumped the diagnostic box and came up with a 27 code. It says its a HSO2 rear circuit. Is this just the rear oxygen sensor, I believe it has 2, and do I need to replace it. I have noticed the MPG went way down. Thanks in advance and I am glad to be a part of the 3slow crowd.

 

There are other things that will cause codes like that....bad spark plugs could cause an O2 sensor issue. How do the plugs look? You might think about a tune-up. Maybe the code will go away then.