Ditto on the pots, but I don't think the diagnostics tests the pots.  You can set the amp to have the pots at either "instant" edit or "last setting" edit (the manual explains the difference).  When they are set to instant edit and a pot is going bad, you can get the constant edit effect while playing.  Try zeroing all the pots, then one by one tweek them to see if or which one is bad, call tech support and they'll send you a replacement (since you changed the battery, you probably have the soddering skill to do the pots).  It may also be the electrolytics and changing the main power electrolytics is pretty straight forward - there are 5 of them, see if L6 can send you them or buy them all for about $15 through digikey.

"Lemmie" is Ken Carver who is a guitarist with the Rhythm Dawgs in TX.  He's an engineer (either computer or electronics - I don't remember).  He left the forum cuz L6 made one too many format changes and it was too tedious for him to stay (or maybe he still watches it under a different name!).  He still offers advice to people about the AX amps from time to time, as the techs at Savage Amps talked with him when they were trouble shooting one of my AX2s.  But I think he's moved on to the Vettas at this point.