If you are using autofuel, the fumes from the autofuel will turn the
cardboard detectors black over time.

Jay Honeck wrote:
Many of us cold-weather pilots worry about carbon monoxide in the cabin,
especially in winter. After using the "dot-that-changes
color-on-a-piece-of-cardboard" carbon monoxide detectors for the last 9
years, we decided to look at the new battery-operated detectors.