I have a panel mounted CO monitor in my ES which I bought from CO Guardian
(http://www.coguardian.com/).  Shortly after I began flying, it started
going off alot, to the point that I suspected there was something wrong with
it.  I saw one of CO Guardian's reps at an AOPA Convention and he gave me a
new unit, no questions asked.  I swapped it out and returned the old one.
The problem went away and I only got occasional warnings when slow with
flaps down.

Next came the paint and interior shops.  Once I got the plane back in the
air, the frequent CO warnings returned.  I spoke at length with the same
company rep at Oshkosh this past summer.  He told me that the sensor in the
units can become contaminated by certain chemicals, including many found in
a paint shop.  Again, he offered to give me a new unit and said he'd have
the sensing element from the old one analyzed.  When he got the results,
they confirmed that some stray molecules from the paint shop (I don't recall
what they were) had contaminated the sensor, leaving a much lower threshhold
to set off the alarm.  The new unit I have now works great and I rarely if
ever have any alarms.

There are multiple lessons here:  First and foremost, take any CO alarm
seriously, even if you suspect it's bogus.  Next, if you have repeated
alarms after thoroughly going over your plane and plugging up any suspected
exhaust entry points, try another detector.  In my case, I hooked up my new
units while the old ones were still installed and confirmed that the older
one was going off while the new one wasn't. Finally, be very careful what
you expose your detector to.  If you're doing any spraying with any
chemicals near your plane, it's a good idea to remove your unit until you're
finished.

Skip Slater
N540ES