user43808: As they say in https://www.arduino.cc/en/Hacking/DFUProgramming8U2 you need to solder a 10k resistor in order to reprogram/reset the MEGA8u2 chip on Arduino UNO but sometimes you may lack the needed tools.
Is there a way to reprogram the MEGA8u2 using an external programmer or an another arduino as a programmer?
My idea behind this is to use the ISP LAYOUT for ATMEGA8U2 in order to load the correct bootloader:


user4143: Sure you can. Solder in a 6 pin header and you can program the ATMega8U2 using any Atmel hardware programmer. It's just a standard MCU, after all.  I'm not sure how the DFU side of things works - whether it's built into the chip in ROM or not1, but if it's just a bootloader in flash your programming may wipe it out stopping DFU from working in future. Not a problem if you can't get into DFU mode anyway.

1: The DFU is firmware stored in the "boot" region of the flash. As long as you don't erase or overwrite that region of the flash DFU will still function in future. Of course, you can also re-install the DFU firmware at any time you like.

43808: <p>As they say in <a href="https://www.arduino.cc/en/Hacking/DFUProgramming8U2" rel="nofollow noreferrer">https://www.arduino.cc/en/Hacking/DFUProgramming8U2</a> you need to solder a 10k resistor in order to reprogram/reset the MEGA8u2 chip on Arduino UNO but sometimes you may lack the needed tools.</p>

<p>Is there a way to reprogram the MEGA8u2 using an external programmer or an another arduino as a programmer?</p>

<p>My idea behind this is to use the ISP LAYOUT for ATMEGA8U2 in order to load the correct bootloader:</p>

<p><a href="https://i.stack.imgur.com/NjHBQ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NjHBQ.jpg" alt="Possible pin layout"></a></p>
 4143: <p>Sure you can. Solder in a 6 pin header and you can program the ATMega8U2 using any Atmel hardware programmer. It's just a standard MCU, after all.  I'm not sure how the DFU side of things works - whether it's built into the chip in ROM or not<sup>1</sup>, but if it's just a bootloader in flash your programming may wipe it out stopping DFU from working in future. Not a problem if you can't get into DFU mode anyway.</p>

<hr>

<p><sup>1: The DFU is firmware stored in the "boot" region of the flash. As long as you don't erase or overwrite that region of the flash DFU will still function in future. Of course, you can also re-install the DFU firmware at any time you like.</sup></p>
