Got an odd issue with my Duet gas dryer.  Clothes have been taking a while to dry and did not seem all that warm coming out of the dryer.  Opened up the front panel and observed the burner.  When heat is called for it clicks, about 10 -15 seconds later another click and the igniter comes on. Gas vale opens and burns nice blue flame - for 20 seconds then cuts out.  Repeats over and over with a time out between cycles.  No vent blockage because it's not vented.

Being it's now June and cold weather is hopefully behind us  ( it is 45 degrees now...) I did 3 or 4 loads of bedding, blankets, sheets and coverlets.  i always clean the light filter when I start a dryer load, but rarely when I do several loads of laundry sequentially.  Turns out one of the blankets shed a lot of fiber and the lint filter was semi packed with lint.  Cleaned it and all is well.

Most of my stuff is years old and sheds very little lint anymore so it's rare to find more than a trace of lint on the screen, have to make a mental note to check between load with bedding.

As I mentioned after cleaning the lint filter all is well. 

As to the CO2, not an issue.  I have both a smart co2/smoke detector and a CO2 alarm with a digital readout and it shows no carbon monoxide.  I'm sure the high output burner on my gas cooktop puts out more co2 than the dryer.  I'd gladly vent it, but it's placement makes it difficult.  Truth is I dry only a few loads a month.  All my dress shirts I put on hangers to dry as well as my jeans and most pants.  Towels and bedding do go into the dryer but I wait until I have a full load and as I live alone it can take some time to accumulate.