Arduino UNO 5v dead shorted to ground.. fried ?

Development Tools IDE 1.x
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I replaced my i2c wiring with a shielded cable, in the midst of that i created a dead short of 5 volts and ground.

I was powering up via the usb plugged into a usb cigarette lighter adapter in the car.

I knew immediately it was afowl as it didnt fire some relays in a certain order on bootup, was plugged in about a minute.

I immediately suspected and found a dead short and removed it.

I plugged the uno into my laptop and it saw it and let me load a sketch.

In that sketch I added Serial,println("L") in loop

I then opened serial monitor and nothing.

The unit is not firing some relays on setup that i fire just to know its booting (they turn some fans on high).

It has 4.75 volts on the 5 volts, i can load a sketch, but it appears to not actually be executing code at all as proven by not firing the relays in a certain order and not providing any serial output via loop.

I saw it has a poly fuse but i am going to assume that if that had been activated i would not have been able to load a sketch ?

Thanks

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update. the uno has a mux ii on it, i pulled that uno and placed another one there and it fired up, so the mux ii works and the old uno is damaged.

i brought the uno in, the serial port is working i failed to wait long enough last time, but the mux ii no longer functions on that uno so it fried something. the damaged uno does run code and has a lite and tx rx lights, but its toast as far as using it with the mux ii. ill trash it, ordered 3 more from amazon.

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Out of curiosity, what's a mux ii