I have a 2013 focus st that I just rebuilt due to a hole burned in the piston. Got the engine work done and reassembled but now when I attempt to start it, it will run for a second and then cut off. Occasionally it won't even run and just backfires out of the exhaust. Took it to Ford and it has no codes and they don't know what's wrong. It has an aftermarket intake and exhaust and did have a Ford performance tune but I took it off after having the starting issues.