The hallway fire detector recently told us it scented carbon monoxide, but we hadn’t been doing any cooking or baking (in the kitchen two rooms away), the furnace (in the basement) hadn’t run for a few hours, and nothing else looked like it was on fire. I had recently replaced the alkaline batteries after a similar false alarm a few weeks earlier; it seems the detector failed after half a dozen years or so.
Tearing it apart revealed something resembling an 18650 lithium cell:
Which made no sense, given the circuitry.
A casual search shows a Figaro TGS5042 is actually a carbon monoxide sensor. I’m mildly surprised enough gas gets through the vents fast enough to produce an early alert:
I tore it apart to reveal a few droplets of whatever the electrolyte might be, so it hadn’t completely dried out.
The Product Information flyer doesn’t define what “long life” might be, but another page says “10 years”, so apparently the rest of the circuitry failed around a not-quite-dead-yet sensor.
The Smell of Molten Projects in the Morning 
Ah, the banshee wail of a failing CO sensor. They have to fail noisy, or no-one would ever replace them. The ones I’ve used claim an 8-10 year life, but have an evil habit of expiring at 1:30 AM. I change them every seven years to avoid that
Failing in the middle of the night must have something to do with slowly falling temperature. I’m sure it’s pure confirmation bias, but it seems their favorite time to howl starts just after oh-dark-thirty.
Having this one sound off before bedtime was a delightful improvement!
I tried one in the shop/barn, but I keep the minimum temp at 38F. (I override the 50F min setting on the kerosene heater and use a wood stove when I’m working in there in the cold.)
The detector was screaming until the place heated up, so I switched that one off and demounted it.
The barn is leaky enough to keep CO accumulation a minor concern. Note to self; get a simple smoke detector.