What do you do if your nighthawk carbon monoxide dectector says error it beeps evry 20 t0 35 seconds

If the display shows "Err", push and hold the Peak Level button. Does the display change to "E09"? If so, then you have just been "had" by the grossly under-documented "planned obsolescence feature" called, rather matter-of-factly, "End of Life". This means that ~7 years after first being powered up, the unit will, BY DESIGN, cease to function, forcing you to buy a replacement. Would have been nice of Kidde to include THAT warning on the OUTSIDE of the package. Call their "Hotline" at 1-800-880-6788 and tell them what you think of that policy. If it is still under warranty (5 or 7 years depending on date of manufacture) they should replace it. In any event, buy another one; if they replace your first one you will have twice the protection. Not a bad thing. *Disclaimer: I am not a lawyer, despite the handle. Words to live by: "Every creative act results from a sudden cessation of stupidity." -Edwin H. Land, inventor of the Polaroid Land Camera"The problem can always be solved with the materials in the room." -My college Physics Lab Instructor