My OBD2 Died :(

my obd2 died :cry:
my car keeps cutting out after running for about 15 seconds,
so i took it to the dealership and they tried to say
my warranty was void because i added a second battery.
i did this over a month ago, it would have messed up then.
they also said that obd2 was not sending any codes even though the light was on, so i took it to another shop and they said the same thing. obd2 was not sending any codes.
what the heck do i do
just start pulling things off and replacing them or what
thats gonna get expensive
or should i just start callin mitsu and try to have a warrantydisagreement suit with the dealershipor what
some ideas please :?: