Hmm, it seems like a lot of us are having problems. My Kidde CO dectector is less than a year old and it's already showing an error. Call Kidde at the following number, Mon-Fri, 8am-5pm, EST.
"The next time your carbon monoxide alarm has ERR on the display, push and hold the Peak Level button.You will see an E and an O followed by a number.Contact our hotline at 1-800-880-6788 and provide us the error code, along with the model number, assembly number and date of manufacture from the back of your unit.If your unit is under warranty, a replacement will be mailed to you."