:1zhelp:

They removed the plugs, cleaned and regapped them, etc.... Ever since then the motor has been pinging intermittenly.

This sounds like the key point from your post. Have you or the dealer(s) rechecked the plugs since that service? Your 2001 should have platinum plugs that should not be cleaned or regapped, plus the fact that they aren't due for replacement until 105k. I'd suspect damage to one or more of the plugs since it sounds like they treated them like conventional plugs.

As far as the OBDII reader, I've bought one in the past that didn't work with my Pathfinder, but now have an Actron reader that works just fine. Not sure how that is since they are both OBDII compliant but it sounds like you have a similar case since the dealer can connect OK.