UNO not working

Todday i went to upload a sketch to my UNO r3 it didnt work on com4 which it normally does so i tried com3 and it uploaded but i got thos error under it:avrdude: stk500_getsync(): not in sync: resp=0x30

the sketch did not work. i never had problems before, but now i cant load any sketch

Back up a step - try the "loopback test" and see if your PC is comminucating with the USB interface on the board.

I think that I got the board working again, but it keeps changing between COM3 and COM4

Sounds like a connection problem somewhere.

Maybe it depends on what other devices you ave connected to your PC at the time. The Comm ports allocated will change depending on resources required. It shouldn’t matter what port is used as long as you known WHICH port has been allocated to the UNO’s USB chip. Its easy enough to check via hardware management (OS dependant).