CO2 (carbon dioxide sensor) Tile showing just a question mark...

Please see this KB/FAQ entry... I believe it applies to this situation...

Question mark "?" shown on Virtual Switch or other connected Thing Tile

The ? indicator from a connected Thing generally means it is not getting an expected Attribute Value from SmartThings. In this case, the Device Type Handler you are using probably hasn't set a value for "attribute switch".

Login to the IDE, check under "My Devices", open the detail page for the Thing, and check this section, "Current States:

If you don't have a value for Switch that is "on" or "off"... then ActionTiles is going to show "?".

If it's a Water / Moisture sensor, we recommend trying to give it a little manual initialization by wetting across the contacts with your finger or a damp cloth. That will usually force an initial value to be sent to ActionTiles. This may be applicable to other Sensors too.

Otherwise... Please check with the author of the Device Type Handler or SmartApp that is associated with the Virtual Switch. Perhaps a value hasn't been initialized?

This is what is in the current states section;

I don't have a 'switch' option at all.

That output is really helpful, thanks!!!

• In the KB/FAQ that I pasted "switch" is just an example. It is a pretty common example because folks use Virtual Switches that often don't have a Current State Value set for the "switch" Attribute.
• In your specific case, you're looking for a value for CO2 (carbon dioxide level). Click that reference, and you'll see that the SmartThings documentation says the the Attribute for that Capability is: "carbonDioxide: NUMBER".
• Notice your "Current States"? There is no Attribute Value set for an attribute named "carbonDioxide"!

ActionTiles can't display a value on a Tile for a Thing (Device) that doesn't have a current Value.

What does the SmartThings App show?