On Sep 19, 9:11*pm, 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?

AC operated? Therefore not low battery, like smoke alarm. Or is there
a battery backup?
Knowing nothing about them except we have one plugged in near the
bedrooms for last few years would one have to careful about damaging
something by too much of blast of say air?