A month or so ago I had a carbon monoxide problem in my house, picked-up by my Nighthawk detector (registered from 10-45 depending on where you were in the house). I had two different certified serviceman come out to check my natural gas 1989 Carrier Weathermaker SX . Each came to a different conclusion. One believed it was solely a venting problem, the other said I also had a bad/plugged secondary heat exchanger.

The exhaust flue was in a corner directly above a basement window and directly below the an opening to the kitchen stove exhaust fan on the side of the house.. The Nighthawk detector registered as has a 45? when placed in kitchen. The first serviceman said move exhaust flu and you will be fine. What concerns me is the second serviceman said my secondary heat exchanger is bad or plugged because the exhaust outside smelled bad. He thought the carbon monoxide in the house may also be caused by a bad seal which must be leaking and also distributing carbon monoxide throughout the house. It's true if you standout side when the heat is on and running there is a very very distinct smell I can't describe, but it is definitely there. He says because the fresh air intake to the system is right next to the dryer vent, it sucked in lint from the dryer and has plugged the secondary heat exchanger. I extended the exhaust flu up above the facia of the house and have had carbon monoxide problem again (no reading on the Nighthawk device). However, here are my questions

1) Is it a problem that the exhaust from my heating system smells really funny and would that indicate the secondary heat exchanger is plugged?
2) If yes, would that produce a higher concentration of carbon monoxide which might have contributed to my problem and should I still be concerned?
3) If yes, is a secondary heat exchanger serviceable? This serviceman said, let just replace the system because it is 15yrs old.
4) What other signs might confirm my secondary heat exchange is plugged? For example, if I cover the fresh air intake pipe it doesn’t seem to be sucking much air. It barely holds big leaf to it, if placed in front. Is that an indicator?

Simple I don’t want to have to replace the system unless I have must.

Your advice is very much appreciated