No idea what lies behind your issue, but regarding the OD501 code ..... it is what it is, and exactly how you have shown it in your post above.
There are lots and lots of codes, for example OD502 is -
Problem Explanation: OR test has failed and Appointment Not Required. Resolution/Recommendation: Please report fault to BTW for OR investigationYou will see that an appointment is NOT required with this, as opposed to one being needed on the OD501. It would suggest an issue may lie within the house, perhaps