Reply To: 2009 Subaru Legacy won't start after head gasket was resurfaced

[Richard Kirshy]

Considering you did have the engine installed and running then removed it a second time when this fault occured.
My first thought would be:
Inspect and double check all your electrical connections are fully seated and are in the correct locations.
Check for any wiring that may have gotten pinched, damaged or broken during the engine reinstall.
Confirm the firing order and timing.
If all those check out, then check for any stored codes.
Check for spark and fuel during and after the engine tries to start.