If there is an issue with the unit, typically it would show an error code "E2"
If your unit is showing fluctuating results by the day, it could be due to the delayed effects by what you've eaten.
Small changes (eg. eating small amounts of food, drinking something) can also cause you to come out of ketosis easily. This is especially true for people who have just started a keto diet since their metabolism has not been fully adapted to use fat as body energy so then the body might use glycogen instead.
It is also better to refer to the long-term changes rather than the short term changes during the day.
However, please note to not perform the test immediately after the following activities:
- Brushed teeth
- Drank alcohol