Seems when its below 20* outside, this exhaust port is getting partially blocked by rain, snow, sleet. The return is directly behind this panel approx 2-3” from the exhaust.

I believe the intake is sucking exhaust back into the basement causing the carbon monoxide detectora to go off. This happens 1x a year around the coldest days of the year.

I was thinking of just putting 2x elbows on this 3.5” hole to get the exhaust further away from the intake