Hi - I am always happy to chime in on this issue although I while I think I have gained an increased understanding of it through the years, I have never solved the problem. I have two 406 heaters, one that heats the pool to 82-85 and one that heats the spa to 104. The pool heater is generally not a problem - it only throws off hi limit 2 errors when the filters are dirty and a back wash and/or new DE fixes the issue (although I tend to forget this at season start-up some years). The spa heater chronically generates hi limit 2 errors as it approaches 104 degrees. It is possible this is flow related, its is possible it is sun related (problem is worse depending on how much direct sun heater is getting) and it may be insulation related. I thought initially the insulation described in the threads referenced helped, but over time, I remove the insulation as I decided it did not help as I initially thought. I often need to reset the heater 5 or 6 times on the way up to 104 (and to get spa to 104, I need to set heater to 106 by overriding its 104 limit). I have changed the components - the internal bypass, unitherm governor and limits themselves to no avail.
One other theory I have is that the problem may relate to age. It is possible that the flow within the heater has been impacted by calcification that I dont see and cannot remove. That may cause a chamber to get hotter than it should. I have no proof here - just a hypothesis. But I do see how flow affects even the pool heater. I can say that turning up my viable speed spa pump and inserting a brand new cartridge filter have not made the problem go away.
One other comment that I completely do not understand is why some people's heaters self-rest while others require the user to hit mode to reset (I have the latter). My heaters were purchased in fall 2010, so they are older. But the circuit boards in each heater have been replaced several times (most recently 3-4 years ago) so I have relatively new firmware.
