2007 mustang will not start

When I first start the car it runs for about 30 seconds then turns off. When attempting
to start again I get the following message fuel level low (it has more than half tank) then
tells me that it's checking charging system. Had a lock smith check and he told me
that the computer was reading the key code so nothing wrong with key (tried both sets
of keys with same result.
Please help.
Thanks
Gilbert