regeneration process

is there a desription of how the regeneration process works? what criteria must be met for the passive regen to occur?
its tough for me to get a good decription from the dealer, the service writers dont know answers to the specific tech questions i am asking.
my trucks are giving me so much trouble this year. i have 7 2011 penske lease trucks with d13
and we own 2 2012 volvos with d13.
they are killing me this year. without a good understanding of how the system works, i cant tell these guys what to do in the breakdown periods.
they call me with codes and we just keep bringing those trucks in, one after another. injectors, DEF pumps, differential sensors, veturi tubes, harnesses, def lines cracked etc.
when a guys gets a code 2659 fmi 16, the dealer says it will go away when it warms up. the problem is, it will not regenerate passivly. this means that SCR shutdown is imminent. truck runs for a while just fine, but once it sees the filter is dirty and tries to regen, it cant regen and the SCR shut down in 60 minutes comes up.
I just dont know what to tell these guys because im not clear on how the system operates.
help would be great here.
thanks.