Fire department has handheld CO detectors that are only a phone call away. If you are unsure about it please call.
We moved into a new house and installed a CO detector that day. A few hours later it went off.
I suspected it was a bad detector but I called the fire department and asked if they had a unit and if they could quietly send a guy over .
Of course, they sent a fire truck and turnout gear. I guess they needed something to do.
I was right. Readings were zero. It was a bad detector.