I am using an Arduino Uno R3 to send instructions to a serial monitor in a Python program with Serial.println. But when I send a specific line, I get a a UnicodeDecodeError.

The line in the Arduino code that causes this error is:

Serial.println("Press '1,2,3,4,5' to run motor at 10%, 20%, 30%, 40%, 50% for 2 seconds");

My Python code to read the serial data is:

def get_serial_data(self):
        while 1:
                serial_data = self.serial_connection.readline().decode('Ascii')
                filtered_serial_data = serial_data.strip('\r\n')
                self.serial_monitor.insert(END, filtered_serial_data)
            except TypeError:

and is called from a thread:

thread1 = threading.Thread(target=self.get_serial_data)
        thread1.daemon = True

But I get the following error:

UnicodeDecodeError: 'ascii' codec can't decode byte 0x85 in position 36: ordinal not in range(128)

Can anyone help why this is happening or how to fix, I saw a stackoverflow post to use a try, except block to get it to ignore this line, but I want to print it out, and I want to understand why I get this error.

  • 1
    As a newcomer to the Arduino community, it would be good to know why this and my answer got downvoted. It was a genuine issue that took me a while to solve, and wasn't obvious from the docs. Without any reasoning for the downvotes makes this community come across as unfriendly
    – stanley
    Aug 2, 2023 at 2:01

1 Answer 1


I solved the issue. It was a memory issue not a decoding issue. Because the dynamic memory was mostly used up, it affected the encoding of the data. Reducing the amount of memory my code was using resolved the issue.

I also discovered, that Serial.println("some text") uses a lot of dynamic memory. So by reducing the amount of text I was encoding helped, but also by wrapping the the text in F(), like this:

Serial.println(F("some text"))

Which stores the string in flash memory not sram. Heres a helpful link on the F function:


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