iPhone4 has no picture. However, responds to touch and has sound. How can i test logic board failure?
My phone was also overheating, experiencing reception problems. It looks like a bunch of dirt and grime got trapped all over the inside from the previous cracked screen. 

The solution: I cleaned off all the dirt and grime off of all the parts with alcohol 70%, used rubber gloves to make sure no oil from my skin transferred to the parts, and I used a blow dryer to blow off any dust. Fixed everything. The overheating stopped and phone reception is a lot better!