Background: In Morbihan, 1,316 public housing units were equipped in 2003 with CO detectors with a stated service life of 10 years. The semiconductor-based detector model was stated to comply with the European standard (EN 50291). Methods: To assess the detectors’ functionality, a descriptive study was conducted among tenants and supplemented by laboratory tests performed on new devices and devices removed after four years of operation. During the tests, the detectors were exposed to gases with CO concentrations ranging from 33 to 330 parts per million (ppm). For each exposure, the response times before alarm activation were compared to the provisions of the European standard. Results: Among the households, 387 were surveyed, of which 19.5% [16.5–22.9] reported an alarm. Acceptance of the detectors was good; however, only 39.8% [35.8–44.0] of tenants had read the user manual, and appropriate responses to an alarm activation were poorly understood. The incidence of days with at least one alarm activation increased by a factor of more than 8 between 2004 and 2006. Symptoms consistent with carbon monoxide poisoning were reported on 7.7% [3.1–18.0] of days with an alarm. The circumstances of the alarms were partially consistent with known sources of CO, and a hazardous situation was detected. None of the new detectors complied with the European standard. Test results suggested rapid sensor drift causing alarms to trigger at low exposure levels, as well as sensor saturation, which could explain instances where the alarm did not trigger. These failures to trigger alarms were observed in 18.3% of tests after four years of operation. Conclusions: Evaluating detector performance prior to market release could be beneficial for public health. Educating occupants on the proper use of their CO detectors also warrants special attention. (R.A.)