First of all my Renix had an egr on it . Secondly high NOX is caused by 3 things in this case 1. carbon build up on pistons due to bad o2 2.high temp, high pressure PVC orifice is clogged 3. bad cat. Your popping noise is more than likely caused by a lean misfire, bad o2 , tune up , ect ect . This will cause NOX and damage the cat. If you replaced the o2 did you use Jeep Parts? If not it is probably already bad as the Bosch and Denso do not work well in the Jeep at all. The quick fix and probably the best is to replace the o2 with factory part and replace the Cat. you can get them from NAPA cheap. Walker part number is a 80905 if you have a 1995. You should also replace the PCV orifice or at least be sure its not clogged.