is the check engine light on when it runs for those few seconds ?
did anyone see if there are any codes stored in the ecm/pcm ?
I am not saying that the ecm is good or bad at this point and it certainly could be an issue ..
I dont understand how you can get a 43.3 volt reading from a 12 volt system .
and that 4.5 volts with the key off bothers me
this is a reference voltage sent from the ecm with the key on ,with the key off it should be 00.0 volts .
here is a diagram of the antitheft system
constant bat voltage on A1-red wire
key on volts at
B2 and A2 pink /black wire -there should be no voltage with the key off
check for grounds at
B5 AND B6 -black/white wire
that blue wire at A3 should not have any voltage reading when it either unplugged from the pass key decoder module orwith the key off
but should have 4.5-5 volts key on .
also ,please see if there are any codes in the ecm and as another thought ,if there is a code relating to the mass air flow sensor -it could be that the maf sensor is bad or dirty -this is just an extra thought -I have seen these fail and give the same problem as an antitheft issue -where they will start and stall after a few seconds