If the DO readings emit a reading of 0 mg/L then:

Make sure the DO circuit board is facing the correct orientation (refer to step 2)

Make sure that the jumper wires are connected correctly and securely. Likewise, check if the RX and TX wires are swapped.

Check the connection between the adapter cable and the housing of the DO sensor to see if it is tight and sealed.

Make sure there is adequate electrolyte solution in the cap of the DO sensor.

If the steps above fail, replace the DO sensor circuit board.

If the DO readings emit a reading of 100,000 mg/L then:

Replace the DO sensor circuit board.