Have an M3 here which just shows a red light when PSU plugged in. I have tried reflashing it, but get the message "COM test error!" "opening serial port is failed".

I know that my null modem cable is OK, as I use it all the time to flash other boxes. I'm also using a pc with a proper RS232 port. I tried using another pc with the same result.

I downloaded all the official abs files and tried them all, with the same results. I have tried waiting various times before plugging in the PSU.

I'm running out of ideas. I think it maybe a hardware fault?