[Galaxy S 4 Mini Q&A, Help & Troubleshooting] mdawdy:
I really do wish some of the more technical guys on this forum would take an interest in this perplexing problem.

Here is a summary of this problem and of the work done by the official repair center, the results, and finally at the end a couple of questions ...

Here is a description of the technical problem with the phone ...

1. The phone will work normally for an indeterminate time period, perhaps a few hours or maybe a week or so. Then it will malfunction as follows -

2. Signal strength is tiny. The phone will not ring for either incoming or outgoing calls. Someone phoning me gets no rings, it goes immediately to voice mail. When i try to call out the screen looks perfectly normal - it visually appears to be calling as the active icon is on the screen and the message says "dialing". There is no dialing sound. It never connects, it eventually gives up.

3. Mobile Data is not working. With Mobile Data enabled there is no icon on the notification bar. Google search and PlayStore do not run.

4. Wi-Fi is working. The icon is on the notification bar when it is enabled. Google Search and PlayStore work with Wi-Fi.

5. The problem is temporarily cured by restarting the phone. Only after i restart the phone do i get notification about any messages that were left. Then eventually the problem re-occurs.

When i sent phone back the first time this is what they did ...

[202]-Full sw upgrade & phone reset

;sw re-flashed then rebuild to factory default

When i sent it back the second time this is what they did ...

[051]-No trouble found

;RF Calibration performed to enhance RX/TX performance and network connectivity. Passed RF compliance test to meet manufacturer soecifications. Adjusted battery level to optimize power usage

When i sent it back the third time they did exactly the same as the second time. The phone is now working better as i will explain, but it has not been long enough to confirm if the real problem is solved.

I have set up the CleanMaster [CM] and BatteryDoctor [BD] Apps. I have turned off any automatic options in both. I make only light use of the phone - a few calls per day, maybe one Google search, nothing else. I run the 1 Tap Boost CM Widget and the Save Power BD Widget after each use of the phone.

I also have installed NoWallpaper [saves a wee bit of battery power], SmoothCalendar [very nice and very useful] and one of the Clock Apps [with a useful 1 x 2 Widget].

I have not yet disabled the 50+ Apps that CleanMaster reports as harmless to disable as my phone is still under warranty and has this issue that the service people have not yet been able to resolve. But i will be doing that and i highly recommend it to others.

I have had the phone back from the 3rd repair for 6 days now. I am leaving it powered on overnight. At the end of the first day battery = 89%, second day 74%, 3rd day 55%, 4th day 46%, 5th day 31%. That is 5 days of light use. Was 23% this morning so i re-charged it to 100%. I'm happy with 5 days of light use.

I do not have written notes on the previous battery performance. But from memory it used more than 20% the first day, at least an additional 30% the second day. There was not enough to trust it for the third day as it lost more overnight. I was running BD and CM, using the CM Widget but not the BD Widget.

I am wondering when the phone will fail again. I'll post again when that happens as i feel certain that it will. But i secretly hope it will not :)

So, finally, here are my two questions ...

What is the actual, presumably hardware, problem with the phone that they are not able to find?

What did they do to improve the battery performance so dramatically? Did they for example lower the CPU clock rate?