My issue: After starting and warming up my 2001 Limited, during a stop at a red light, the idle will drop and "bounce" between 200 and 500 rpm and at times completely die. As mentioned this only occurs at a red light or stop either with the engine in D or N. It just started about 2+ weeks ago and is getting progressively worse but only after engine warm up. Other than that it drives great with no other issues.
Not sure if this has anything to do with the problem but about a month and a half ago my check light came on. After taking it to AutoZone and having them check it with a OBDII scanner, I was told the code was my CAT converter and should be replaced. A week or so later the check engine light went off. Fast forward another week the light came on again but only for a few days. I never got it checked this the second time thinking it was the CAT again. A few days later it went off and has been off since which now is going on a few weeks. Not long after the idle issue surfaced. I took the XJ to my mechanic about the idle issue and told him about the check engine light previously being on and what AutoZone told me. He tried to "pull" the previous code from the ECM using his OBDII scanner and he couldn't find any codes present.
Is there another way to "find" the previous codes and would a bad CAT have anything to do with the idle issue?
What steps should I take to systematically eliminate items that could be causing the low idle? Should I replace the CAT after what AutoZone told me even though my mechanic couldn't recover any codes? Sorry such a long read but any and all help would be greatly appreciated. Thanks all.

 

My old '01 got threw a weird out of the blue "catalyst inefficiency ___________" code and developed an unstable idle. I gave it a WOT blast and cleared the code which never returned, give it a shot.