I think that I can exclude the exhaust leak then. If there is one it must be extremely small for me not to notice and for the emission testers to not detect.
The C180 only has one O2 sensor and that is in the manifold.
I am not sure if it has an EGR. I cannot recall any pipe feeding from the exhaust.
Again as part of my continuing education, do my emissions reading indicate that I have both high O2 and CO content in my emissions? If so, does this mean that the other exhaust gases (CO2, NO, ) are significantly under-represented?
I wish I could ignore the demons but as you will be aware these sorts of problems can eat away at you as you keep searching for something that you've missed.