Nixie tubes suffer from a specific problem called cathode poisoning. Ideally, all digits (cathodes) in a nixie tube should be used regularly. However, that is not true for a nixie clock. Only even tubes run in this optimal mode, odd tubes use only some digits. For instance, the leftmost tube uses digits 0,1,2 to display tens of hours position. The third tube (tens of minutes position) uses digits 0,1,2,3,4,5. When a particular digit is not used for a long time (e.g., digit 8 on the leftmost tube), that digit is covered by a metal deposit released from the other activated digits. That particular digits will eventually have defects in the glow once you need to use it (e.g., to display custom data).

Problems with the glow can be avoided by regular exercising of all digits in a tube. Just a few minutes per day is enough to keep the tubes healthy. For this purpose, we implemented a routine called Cathode cleaning interval that is regularly activated and keeps the nixie tubes healthy.

If any of the tubes shows a defect of the glow, you can try to Heal it.