That cap, seen through a slit in the edge of the units, covers one of the two smoke sensors and can compromise the unit's effectiveness.
Affected Kidde smoke alarms include models PI2010 and PI9010 and will have the date code "2016 Sep".
The company says all consumers who have this product should remove it from where it is placed, and inspect it through the opening on the side to check if there is a yellow cap. If the cap is present, consumers should contact Kidde to receive a replacement.
Smoke alarm manufacturer Kidde is recalling half a million smoke alarms that may not work properly.
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The affected alarms were sold at Walmart, the Home Depot, Amazon.com, ShopKidde.com and other stores and retail websites nationwide between September 2016 and January 2018.
The recall involves 452,000 detectors in the United States and another 40,000 in Canada.
People are encouraged not to take the alarm part, open the casing, or remove the yellow cap. Do not remove and discard the recalled alarm until you have received and installed the replacement. "If no yellow cap is present, consumers should reinstall the smoke alarm and no further action is needed".