Hi Steffen Wichersheim,
Yes, you can use UART to flash the KL03 MCU, because this chip have the kinetis ROM bootloader.
For details, please refer to the Chapter 11 Kinetis ROM Bootloader in the KL03 reference manual.
Wish it helps you!
Have a great day,
Jingjing
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------