Why is my MacBook turning off?

I have a MacBook Pro that I recently replaced the logic board in. The logic board I installed should be fine (absolutely no liquid or any other damage). When I turn the MacBook on, it comes on normally with the chime and the screen turns on, but the fans race at full speed. It starts to load into OS X (Apple logo appears) but after about 6 seconds it shuts down. I've tried the pram/smc reset to no avail. I was hoping to check if the logic board was covered by AppleCare but there isn't a serial number on the board. Any ideas how I might troubleshoot the issue? Any words to the wise are appreciated!