Last time our wife and I had a tune-up on the furnace, every one of us found out that the carbon monoxide detector was defective, and it was truthfully one of the scariest things that I have ever heard, but the heating and a/c service professional performed a full evaluation on the furnace, and when she checked the carbon monoxide detector to make sure it would beep, nothing happened.
The woman even changed the batteries, and the carbon monoxide detector still would not work, however at that point, the heating and a/c service professional proposed us to purchase a new detector, but my wife and I were beside ourselves with horror.
At any time the furnace could have started leaking gas into the air and our family could have been killed, but every one of us instantly bought a new carbon monoxide detector from the contractor. The woman installed the machine at no charge, because every one of us were having a tune-up service performed. That is one good thing that I like about the contractor and the yearly tune up service. If the service professional finds a problem and every one of us have it fixed at that time, they do not charge us any extra labor. Every one of us only get charged the cost of the parts that are necessary to fix the problem. This was truthfully one of the first times that every one of us ever ran into an issue. The professional assured us that there were no troubles with the furnace and the glitch with the carbon monoxide detector was simply due to the age of the machine. I still suppose every one of us need a back up system in the basement.