S4 - Not reading SIM card; therefore no phone
The S4 is fairly modular and with some guidance made this effort very easy. The diagnosis was the difficult part and first spent $4.00 for just the reader. The replacement motherboard had one on it. Replaced the motherboard in about 1/2 hour and unlocking from AT&T took about 1 hour (via email). So happy.
Don't be afraid. Go slow and research. If you need to start 'guessing' with replacement parts, go from cheapest and more expensive.