Jim wrote:
Came home today to discover that my First Alert model FCD4 was acting
strangely. Single chirp every minute or so. Looking closely at the
display was the "err" message.

I'm guessing it is one of two factors. #1 being they are cheaply made
and simply don't last more than a couple years. #2 being the sensor is
dirty causing the error message. If so how does one clean it? Would
compressed do?

Did you contact First Alert support?
Did you RTFM? Always keep the manual.
They know more about their product
than I do.

http://www.firstalert.com/support.php

[8~{} Uncle Monster