Since i have a non flued Gas heater in my house I purchased a CO sensor alarm on Ebay $20 delivered. On pluging it in it didnt work so contacted the suplier & they would repay the money. I then decided this wasnt realy what i wanted to do as i still needed a sensor. I prised it open & found the power transformer in side had damaged conections on the copper tracks for the 12v side of things. Repaired the fault & decided to check if this thing actualy works. Went out to my workshop & set it up inside & ran a Generator 3Ft outside the door with the exhaust facing the door direction. After a few minutes the alarm went of. Now satisfied that it does work.