6.8 mpg, check engine light, NSS adjustment, 98 XJ

[McQue]

My exhaust manifold has 2 sections...first section takes care of 3 cylinders, and rear section takes care of 3 cylinders...and there is an 02 sensor upstream and downstream in each section with a mini-cat in between them.

 

[over2land]

Its the two piece cast one?

I had a buddy with a TJ with (likely) the same setup.

00 or 01 TJ.

 

[McQue]

Not really sure...I have not had to take it apart yet, so I don't really know if it comes off in 1 or 2 pieces if that is what you are asking...it does have 2 individual "down pipes" with a min-cat on each that are connected to a "Y" like pipe into one pipe on the next section of the exhaust...

 

[UNCC_99XJ]

I was behind a chrysler suv, forget the model, looks like a cross between a durango and a pacifica, it was on a car hauler, I counted two extra O2 sensors, one in front of the muffler and one on the tailpipe, whats up with that...

A lot of them are like that. The KJ 3.7 for example has 4, much like the 00-01XJ's.

McQue is correct in his post. This is because there were 2 more oxygen sensors added in 2000.

So, to recap. Pre 2000 = two Oxygen sensors. One upstream, one downstream. The upstream controls air/fuel ratio, the one downstream mointors the converter.

2000+ there are 4 oxygen sensors. All 4 control air/fuel ratio, with the downstream ones working in conjunction with the upstream.

 

[Timber]

McQue, an '01 is much different.
The P0171 was definitely related to the O2 sensor, as you figured out. From now on, try to search for the P-codes if you can. I had a P0171 problem a while ago, and searching for the code might have gotten you the answer a bit faster.