https://www.planeandpilotmag.com/news/pilot-talk/2021/11/24/a-neglected-172-turns-toxic/

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The Board concluded, “…it is most likely that the pilot experienced carbon monoxide poisoning during the flight when carbon monoxide entered the cabin from the degraded right muffler. The carbon monoxide poisoning led to the pilot’s impairment/incapacitation and his inability to control the airplane.”

Because of post-accident fire damage, the NTSB could not determine if the airplane was equipped with a cockpit carbon monoxide detector. One popular online pilot shop I checked sells several types. The cheapest one is $4.95.