My 2000 xj w/4.0 just passed state inspection then after 100 miles or so engine light comes on again . Here is what I did before inspection.

New front pipe
New rear Cat
New muffler
New tailpipe
All were walker brand
New o2 sensor bank 2 #2 NTK brand
New NGK spark plugs

Engine runs smooth no other issues, did a 100% cooling system replacement with exemption of heater core.

Where do I look next ?? Do I replace other o2 sensors ? If so which ones or remaining 3 ? New mini cats ? Is there a vacuum leak or misfire that could cause code ?
Looking for repair that wont break the bank

 

Its a good idea to change all the O2 sensors because as time goes on they can get lazy, but your problem is PROBABLY the mini-cats. You could try making sure the flanges are tight first, but start planning on dropping the money to replace them. It can be difficult to get at the bolts. They are 15mm and require a universal and low profile socket, or maybe a swivel socket with the universal built in.

 

Have you tried a blast down the highway at WOT? Sometimes after a lot of creepy crawly city driving that's the best way to restore efficiency.

 

Having the same problem in my '00, Mine was fine till I decided to fix things that did not really need fixing. I upgraded to 4 hole injectors and swapped out all four O2 sensors. Justified replacing the sensors by thinking they were original, had 150K on them and needed to go. Used NTK's. So then I get the P0432. Best I can tell my fuel trims are OK. Tail pipe is clean and no weird smoke coming out of it. My O2 sensors appear to be OK too. I was getting a pending P0456, small leak evap code, after I did this. Replace a couple of the rubber elbows on the intake manifold and threw in a new gas cap for good measure. The pending code went away. But the P0432 still came back. I could not find any exhaust leaks but when poking around saw that one of my rear exhaust manifold bolts was backed out about a 1/4". Got up in there and torqued them the best I could. P0432 came back. Funny thing is that each time I tried something it took longer for the P0432 code to come back. When I looked at my freeze frame data each time it said something similar to this:

P0432.............O2S 2/1 Catalyst Efficiency
RPM 640
Calc load 5.1%
MAP ("HG) 12.7
Coolant 201degF
STFT1 -2.3%
LTFT1 1.6%
STFT2 -3.9%
LTFT2 0.8%
VEH speed 0MPH
Fuel sys1 CLSD
Fuel Sys2 CLSD

I was noticing the MAP number. Seemed a little off to me. Not going to lie really do not really understand that. Thing is that according to Mopar the cat gets tested under these conditions:

P0432-2/1 Catalytic Converter Efficiency
When monitored: After engine warm up to 147'F, 180 seconds of open throttle operation, at a speed greater than 20 mph, with the engine @ 1200-1700 rpm and MAP vacuum between 15.0 and 21.0 inches of mercury(Hg).

So I am wondering my my freeze frame data is at an idle. So now I replaced the little elbow under the MAP sensor. $4 from Jeep. Waiting to see. Thinking next I might just throw a new MAP sensor at her. Probably will be a waste. Then I am going to replace my new, well not so new now, rear lower O2 sensor. Would not be the first time a new part was defective out of the box. Even though I do not think that is the problem either. Been saving for the new cat like md21722 said because I am running out of ideas. Also what Turbo X_J said. Some say that driving it too little does not let the cat get hot enough to burn off what deposits might be in there. I live a 1/2 mile from work and have no life so the Jeep does not get driven much. Actually go out of my way in the morning on the way to work to drive it a bit. Every once in awhile take it for a good blast on the highway. The only reason this is not pissing me off more then it should is that I happen to need a new muffler and tailpipe. Do not see why that could be the problem though. Just holding off on that if I am end up replacing the cat. Figure easier to deal with the whole system at once.