Which rom?
Usually the rom comes with a kernel that includes the correct recovery -
- Extract the boot.img from the rom zip that you want to flash.
- Flash that with fastboot or use the fastboot mode in Flashtool if you're not comfortable with using the command line.
- Boot into recovery and flash the rom.
Then explain yourself better in the first place. You said the same effect. This is a different question altogether.
Just flash a stock FTF using Flashtool and exclude everything but the kernel.
Yes, you can.
Edit - There's nothing wrong with the PM system. I'm just not sat here all day waiting for people to PM me questions.
It depends. If your bootloader is unlocked, you need to relock it to update OTA. Sometimes if you modified /system in any way (like deleting system apps, flashing mods etc), an OTA will fail.
Lock your bl and check again then you can search what is what
Sent from my C6603 using xda app-developers app
No build.prop tweakes done?
Sent from my C6603 using xda app-developers app
The OTA failed multiple times for me when I was only rooted as well. In the end, I just flashed the update with Flashtool and unlocked my bootloader to root. It was quicker than messing around trying to get the OTA to work.