Code dilemma on '98 XJ
 

Son is home from college for Thanksgiving break with his (my) 1998 Jeep XJ. Getting ready to do a lift on it, and notice the CEL ablaze. Already changed the battery (Oct 2012) and Crankshaft Position Sensor (Sep 2013), so a little lost and in need of help. Pulled the codes and found the following:

P1682 - charging system voltage too low (alternator? PCM?)
P0320 - no crank reference signal at PCM (CPS? PCM?)
P0420 - catalyst system efficiency below threshold (o2 sensor? catalytic convertor)
P0138 - o2 sensor failure

Did a comprehensive forum search, but no luck. Old girl starts and runs fine (even in the cold - 10 deg in Colorado this morn.), but volt gauge is reading 14 at start, then moves to just above 9. No dying issues. I've used the search function, and smacked my head into enough books to last lifetime. May not even be connected issues, but wanted to check first. Thanks in advance!

Cheers,
Pappy

Why not pull the alternator and run it over to Autozone for a quick (free) check? At least it will rule out the most obvious cause.

Good call Roy, and I'm in the process of taking that advice as we speak. I'm new to the Jeep world with little PCM experience, and since the PCM is a probable in both of the first codes and the alternator is only a probable in the first, I wanted to see if people usually found 1682 and 0320 when PCMs were on their way out. Thanks again.

Could be the PCM, but you can get all sorts of strange and seemingly unrelated codes when the charging system is not functioning properly. Plus, I am a firm believer in trying the easy (and inexpensive) fixes first.

True. Ditto for poor battery cable and ground connections.