I took all three CO detectors outdoors and exposed them to exhaust from my ‘87 Dodge pickup. (The new ones have battery backup and I used an extension cord for the older one.) All three alarmed after a minute or two in the smoke. I should mention the ol’ Dodge burns some oil until the engine warms up. I find it curious that the two with digital display would not display any numbers while alarming but I didn’t want to play with them for a long time because it’s cold & snowy outside today.