I had the same experience as Mike191. My MX-850 had a "Fail 5!" code show up during a download, then I tried taking batteries out to reset and the unit started cycling in test mode over and over whenever I hit a key and said RAM "empty". I tried all of the above...hold main+enter, hold Main+stop, take out the battery and hit light button repeatedly, try loading blank program, load minimal program with 10 devices, etc. Nothing worked and I kept getting "Fail 5!" and test cycling. So I took all of the batteries out and left it a couple of nights and this cleared up the issue just like it did for Mike191. When putting the batteries back in Ram tested as "good" and the remote took the same program it had "Fail 5!" on earlier and is now working fine.

Thanks Mike! I was about to toss it and was shopping or a new one.