I’ve seen this before. My wife owned a Cessna 182RG and although the exhaust heat exchanger(s) were leak free there was so much CO getting into the cabin that the colored dot detectors would turn dark within 30 or 40 minutes after takeoff.
We finally discovered it was exhaust gasses from the exhaust pipe there were getting into the cabin through the air conditioning condenser vents on the side of the fuselage aft of the left main gear. Since we didn’t have a clue where it was coming from we were forced to plug off each possible entry point one by one until we found the entry point for the CO gas. We ended up blanking off the condenser vents.
I wish I had a better answer.
Even a small amount will register and since you’re seeing high concentrations down near the fuel selector I’d take a hard look at the firewall seals.
