(American Journal of Public Health) In a study of 30 carbon monoxide detectors in use in the US, more than half were found to be functioning improperly, either alarming too early or too late. 40% failed to alarm in hazardous concentrations despite appearing to be operating properly.
Ryan, T., Arnold, K., (2011) Residential Carbon Monoxide Detector Failure Rates in the United States. American Journal of Public Health, 101, 15-17, https://doi.org/10.2105/AJPH.2011.300274