Brand new Uno misbehaving - stk500_agetsync errors

IDE 1.8.51.0, Windows 10.

I bought 4x new Arduino Uno's to flash together with the same project.
Brand new, never touched, out of the box 1, 2, 3 all flash right away and we're up and running.
The 4th doesn't enumerate in Windows device manager at all. I grabbed my Atmel-ICE and plugged into the ISP port and fired up Microchip studio - sure enough the MCU checks out and looks fine.
I checked the application MCU as well and that also appears to be alive.

I grabbed the Uno DFU and serial hex file from the github project here, and installed the hex file using Microchip studio. I also program the fuses in line with their recommendations to the Atmega16U2 based on the github readme. Everything flashes and reads back fine.

Now I plug my Uno back in via USB and sure enough it shows up in the Arduino IDE - great!
I can query board info and get a response back:

BN: Arduino Uno
VID: 2341
PID: 0001
SN: 8503631353035150C020

Looks good to me.
I go and click program with the same application as before, and all I get are these errors:

avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x55

I've restarted my computer, reset the Arduino, reinstalled drivers, restarted the IDE, used different USB ports, etc etc on and on.

Is this likely just a board that shouldn't have left the factory? Some unseen hardware issue with the Uno?

Check nothing is connected to any pins. If so, check the board selected is correct. Also, have you downloaded the FTDI drivers?

https://ftdichip.com/drivers/vcp-drivers/

It's a brand new Arduino Uno bought from Adafruit, directly out of the box, nothing connected to it.

I can unplug this Arduino Uno, change nothing else, plug in another Uno using the same cable and same USB port, hit program without closing the Arduino IDE, and it works. This to me suggests it's not a driver issue.

Check nothing is connected to any pins. If so, check the board selected is correct. Also, have you downloaded the FTDI drivers?

VCP Drivers - FTDI

I just downloaded and installed the latest VCP drivers from the link above. It did not change the error.

Ok. Have you tried reburning the bootloader?

(Note that the bootloader is a different thing than the 16u2 firmware. It wouldn't surprise me if a board that escaped without 16u2 firmware also escaped without a bootloader on the m328. On the other hand, if the board has a 16u2, the FTDI drivers should be irrelevant.)

I can't burn the bootloader via the Arduino IDE as I'm stuck with the same connectivity issue.

I took out my Atmel-ICE and instead used the IDE and 6-pin header to directly program the bootloader to the 328P - it successfully flashes.

Now I go back to trying to program via the IDE using the Arduino Uno ISP and it works??

Anyway thanks for your help here in salvaging this one!

Burning bootloader only works using ICSP.

Is that a question? Uploading your sketch via ICSP should be successful if you can burn the bootloader; it just bypasses the bootloader. So there is no proof that your issue is actually solved :wink:

Not a question, punctuation of surprise that I was suddenly able to successfully program the application via IDE and ICSP.