How to fix an iPhone that keeps restarting - Panic Log and iDeviceLogAnalyzer
You may find that your iPhone boots up fully but is restarting periodically over a set period of time, most commonly after 3 minutes. The first step to solving this problem is to examine the panic log within the iPhone's settings. You can find this by navigating to Settings > Privacy > Analytics & Improvements > Analytics Data > panic-full-(date).ips
After pressing "Analytics Data" remember to scroll down until you find the "panic-full" field. It can be far down the list sometimes.
Your Panic Log
"Userspace watchdog timeout" is one of the most common hints at this problem. What does this mean? We can find the answer by downloading the iDeviceLogAnalyzer tool. This tool will interpret all the text and tell you the likely cause of your iPhone restarting. To download the tool, simply click the iDevice Panic Log Analyzer field under the "Releases" section as indicated below.
Using the program is very simple. First you plug your iPhone into your computer. Once it is recognized by your computer, the program will automatically connect to your iPhone.
Select "Read Logs" to view the different crashes your iPhone has experienced.
Now you can see the different crashes your iPhone has reported. These should be the same that you saw within "Analytics Data" field on your iPhone. It is usually best to select the most recent panic log.
Once you select a panic log, the program will tell you the possible causes of the restart. For this case the causes are likely "Charging Port Flex", "Power Button Flex", and "Battery SWI".
To further help break down the possible solutions, you can open the 3uTools program. This tool is used for many different types of iPhone diagnostics and is commonly used by iPhone Data Recovery specialists and iPhone Repair shops.
With 3uTools open we can see that this iPhone cannot read any information from the battery. The "Battery Life" field shows "No Data". This indicates a "Battery SWI" problem, which is the motherboard line that interfaces with the battery. This simply means that the motherboard cannot tell how much the battery is charged. When this is the case, the iPhone will shut down as a precaution to avoid any type of catastrophic failure such as an explosion from an overcharged battery.
When you can confirm this problem, your best best is to find a Motherboard Repair Technician to repair the issue. Here at iBoard Repair, we have no problem fixing this issue by replacing the charging IC chip on the motherboard and allowing the iPhone to once again read the information coming from the battery.
A Working Battery SWI Line
Once the motherboard is repaired, we can check with 3uTools to verify the issue is solved. We can now see the real battery charge (97%) and the battery life (aka battery health).
Alternatively, if 3uTools had no problems reading the battery, then you likely have a simpler problem. If the battery data can be read then the problem is likely either the charging port flex or the power button flex, as indicated by the iDeviceLogAnalyzer. When this is the case, you can take the phone to an iPhone Repair Technician to have them replace the charging flex or the power flex. To verify the problem is solved in this case, simply turn on the phone and wait longer then 3 minutes to see if the iPhone still restarts.
If you are an end user, I hope this guide helps you find the best steps to start your repair process. If you are a repair shop owner, I hope this helps you pinpoint the problem and cut down your diagnostic time.