Question: I have a ventless gas log that I installed in my existing fireplace and when I shut down the vent sometimes my co tester goes off. Should I be concerned? My logs have the ods on it. Thanks, Answer: Yes, you should be concerned. The ODS only works when the room air oxygen level falls below a certain point AT THE LOCATION OF THE ODS If your Co detector is working..and is going off, then something is not right. Call your installer, the gas company or the manufacturer.