- Clean the 510 connector on the 521 really well with a cotton swap dampened with rubbing alcohol. Do this 2-4 times for a good clean, using a clean end of a swab each time.
- Also clean the 510 on your atomizers with rubbing alcohol on a paper towel.
If that doesn't fix the problem...
- check it with higher resistance, like say 1 ohm or so, then see what happens. If still says "lo" then you know the 521 is most likely defective.