Arduino Uno not recognized

[nSomnius]

I have a couple Arduino Uno's, one works, the other not. It refuses to show up in the device manager. Are there some tests I can perform to troubleshoot the electronics?

[nSomnius]

Connected via USB cable I am seeing proper +5V and +3.3V at their terminals.

[nSomnius]

Also, the 'L' LED and the power LED are lit and steady...

[nSomnius]

It did finally show up in the device manager but showed an error. I updated the driver for it and it accepted the driver but still won't start. The error message was:
"Arduino Uno
This device cannot start (code 10)"

[adafruit_support_bill]

Sounds like it is communicating at least enough to identify itself to the computer. Did it load the correct driver? Is it showing up as an UNO in Device Manager?

Try a different USB cable if you have one (it's surprising how many bad cables there are out there). Also a different USB port.

[nSomnius]

On the random occasion it does show up in the device manager it does read Arduino Uno. I'm using the same driver I use for the working Uno, same cable, but a different cable does not help, different ports (on the motherboard itself) don't help. Still searching for some diagnostic's to check the Uno with a multimeter if possible beyond what I've done so far.

[adafruit_support_bill]

This doesn't sound like a problem you can diagnose with a multimeter. Was this UNO purchased from Adafruit?

[nSomnius]

Yes, on 21 March, 2012.

[adafruit_support_bill]

Yes, on 21 March, 2012.

So it has been working up until now?

[nSomnius]

It happily ran sketches up until a few days ago.

[adafruit_support_bill]

Sounds like it might be a bad 16U2 (the chip that does USB/Serial conversion on the UNO). I don't have any personal experience with this, but some have been able to revive their UNOs by re-flashing the 16U2 firmware: viewtopic.php?f=25&t=25146

[nSomnius]

That was quite an odyssey. To no avail though. It seems it cannot succeed without being recognized in the device manager.

[nSomnius]

It seems be continuing to run the last program I uploaded. Reset still works. Not sure what else I can try. Open to ideas.

[nSomnius]

Another diagnostic tip I found suggested removing the ATmega328 chip and see if it's then recognized by the computer. That didn't help either.

[adafruit_support_bill]

Yes. I think the problem is that your 16U2 chip has stopped working. If you were unable to re-flash it, you are probably out of luck. If you are skilled at surface mount soldering, you might be able to install a replacement 16U2 and flash that with the firmware. But there is no guarantee that would solve the problem.