Unlock your bootloader again if you locked it, then use fastboot to flash a custom recovery like TWRP 126.96.36.199.
Once you have the recovery installed you can flash a ROM by either side loading it or use a handy USB OTG adapter and a flash drive. There are many guides available that cover all of this already online. Just curious, are you S-OFF?
Hello. I was on TMO 4.4.2 S-On unlocked bootloader. I wanted to downgrade to 4.3 so I downloaded the RUU for TMO. Then I re-locked bootloader and tried to run the RUU with no lock "ERROR 155". I searched for the KK RUU but isn't available. Now I'm stock at the bootloader with the "SYSTEM WARNING". Can someone please tell me how I can either get to downgrade with the 4.3 RUU or how to get my device up and running?
1) Downgrade requires S-OFF. You cannot flash an RUU for a lower firmware if ur S-ON
2) Instead of going through the ugly RUU process (meaningless), you can just install a 4.3-based ROM. Unlock your bootloader. Flash TWRP 188.8.131.52 using fastboot. Then, adb sideload Android Revolution HD 31.6 (4.3) to ur phone and flash in TWRP.
At the end, ARHD rom is basically stock ROM with more tweaks. So, installing a ROM is better than the headache of an RUU
---------- Post added at 09:56 PM ---------- Previous post was at 09:55 PM ----------
Yes, the same unlock code.bin can be used