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First try replacing the batteries.... no, you've already tried that.
How old is the alarm?
Where has it been operating?
If it has been 7 years, the ERR message means it is at the end of its life, and must be replaced as a whole.
Try this: press and hold down the Peak Level Memory button.
If the digital display now shows E09 then that confirms it: your alarm is at the end of its lifetime (of about 7 years in-use.)
Your smoke alarm will not detect CO in this mode. It is strongly recommended that you replace it with a new one.