An "Err" message may appear on your carbon monoxide alarm’s digital display for several different reasons.
ERR or END will appear at the end of the alarm's 7- or 10-year life.
ERR will also appear if there's a sensor error. If you press the peak level button and a code appears (for example, E03), this is the case.
These are "non-recoverable" errors, and the alarm must be replaced. If the unit is within warranty, please call customer support at 1-800-880-6788 Monday through Friday from 8:00 AM – 8:00 PM EST and Saturday 9:00 AM – 3:00 PM EST to get a replacement.