My S4 has the no sim error restart phone problem. Its stock U.K. (BTU) and has never been rooted.
1st occurrence within 3 months ownership but it got to a point, after a week, where it always read no sim even after restart. Samsung warranty repair changed mother/main board.
Phone was dropping calls after the warranty repair so went back to Samsung. Warranty repair 2. Changed 2 wires, don't know which ones. But now phone was working fine.
3 Weeks after repair 2, the screen went black. I could still hear all tones etc but no screen so can't answer calls (most basic function of phone). Samsung put new screen and another new main/motherboard. So now this is the 3rd mainboard. Phone came back on 2/10/14 after the repair. After 2 days I have again got message about 5 times at a time but restart is solving it for now.
So I think the problem lies in the faulty mainboards Samsung are manufacturing. Why? Because my 1st board failed and eventually always had the message no sim inserted. The engineer report said main board was changed. If it was too do with a faulty sim tray then wouldn't they have fitted one of those instead. 2nd board worked fine even though calls were dropping. This board was in phone for 3 weeks until screen got changed so I'm presuming had to change board along with screen. The latest main board which is apparently new has lasted 2 days. Like I said above its only happened a few times so far but I rather it happen all the time so I can send it in again.
I have spoke to Samsung UK escalation and they said if I require another repair then they will take it higher up but even with an exchange what is there not to say new/refurbished S4 wont be faulty down the line.
What is Samsung doing about this, absolutely nothing. My worst ever experience with a phone and I got a s 1,2,3 which all still work not even a glitch. Will buy another phone but keep fighting for a resolution with this one. Still has 17 months warranty.