On Dec. 20, 2021, the NTSB issued Aviation Investigative Report AIR-22-01 urging the FAA to require carbon monoxide detectors in piston-engine, enclosed-cockpit aircraft and asking pilot industry groups to encourage their members to install CO detectors with active aural or visual alerting and proactively ensure thorough exhaust inspections.
The full text of the AIR may be read at https://tinyurl.com/2p8na368