A little further into the device, and we find the CO detector, disguised as a capacitor in a tiny metal barn.

The Figaro TGS5342 electrochemical carbon monoxide detector works like a little fuel cell—the presence of carbon monoxide drives a chemical reaction that generates current proportional to the CO concentration in the air.

The detector's measuring circuitry is on the reverse side of the main board, encased in some clear adhesive—likely to protect it against shock and the atmosphere.