"not in sync" error, "L" LED blinking frantically

I have similar problem on the Nano.

The L “Led” keeps on blinking frantically no matter I try “Loopback”, “Change the Bootloader” etc.

Any help appreciated.

Cheers !!

Hi @anilru. These symptoms can be caused by a deficiency in the bootloader used on pre-2018 official Nanos and even many of the Nano derivative boards to this day. If you trigger a watchdog timer reset, the board goes into a perpetual reset loop.

If that is the problem you’re having, the fix is to burn the bootloader. That will require an ISP programmer. If you don’t own a dedicated programmer, you can use a spare Arduino board by following this tutorial:

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