The fact that you are getting "Check Engine" on when you switch on the ignition tells me that there is a good chance that the "Handshake" between the becm and the engine ecu is o.k.
Have you tried entering the EKA ???
Did you do the headgaskets yourself?? Have a look around the top of the engine bay and double check all connectors are plugged in, including wiring to the alternator..if they are o.k., get underneath and check the connectors to the starter motor and have a look for any others that may be off.
Unlikely that lower connectors would have been disturbed having done the top end, but best to be sure.
:behindsofa: