Fernando, someone else will probably have a better answer for you, but the CO detector that I have requires a replacement of the CO sensor every 5 years I believe. It could be you just need a new sensor. Check it out and good luck contacting Guardian.
I just checked out their web site and they seem to be up and running. Here is their blurb on replacing the sensor and recalibration:
"Recalibration
Re-calibration:
Although the CO sensors in our CO detectors have a life of 5 to 7 years under ideal conditions and if protected from all external
contaminants, we recommend a re-calibration of the units after 5 years. The CO sensor is very sensitive, and certain external
substances in the ambient air can change its characteristics and affect its reliability.
The re-calibration can be done by us for a flat fee ( Please contact CO Guardian for pricing information). This service includes
installation of a new CO sensor, re-calibration, and a complete system check. You get a fresh 12 month warranty after this service."
Sounds like you need to send your unit in for a new sensor and recalibration. Your warranty appears to have expired after 1 year. Good luck!
