I have a forty year old house with a forced air gas furnace. Recently on its yearly start-up inspection by the gas company, I was informed that the furnace passed CO tests with flying colors. The inspection did reveal that the cut-off valve and copper feed line were out of code and would need to be replaced (I replaced them). The inspection also revealed that a gap in the gasket material over the heat exchanger cover had allowed corrosion to occur on the case of the furnace (a Sequoia)- resulting in a small hole in the case not near any vital components. The gas man suggests that this corrosion could also be present within the heat exchanger, though he did not open it up and inspect it. I'm thinking that the furnace case was never meant to be subjected to the warm moist effluent from around the heat exchanger and thus experienced accelerated corrosion which probably did not occur within (as noted by the zero CO output test). Is there some material that I can repair the case with and fill in the gap in the heat exchanger cover gasket to prevent further corrosion? Are my assumptions concerning the probable integrity of the heat exchanger warranted? In the interim I've got a household CO monitor running to ensure safety.
I have several strong reactions to your story:
(1) Testing equipment is great stuff. CO Testing equipment is great stuff. I have a very good CO tester. HOWEVER no matter how great any testing equipment might be, it must be used by a professional who understands it's use and limitations!
There are many reasons why a furnace CO test might pass with flying colors one day, and produce CO the next. For Example: I saw a gas furnace this morning with about 3/4" of rust flakes at the bottom of the heat exchangers. These rust flakes came from the inside walls of the heat exchanger, this suggests that the walls are very thin and may fail at any time. The rust flakes could also be covering up a crack in the heat exchanger. A CO test without a a cleaning and then a careful visual examination of the heat exchanger if of little value. It is possibly misleading!
(2) Why "did (the gas man) not open it up and inspect" the heat exchanger on a "forty year old... forced air gas furnace"? The older I get the less comfortable I am with judging things by their age, but most gas furnaces have a life of about 25 years. So, in human terms this one is well over 120 years old!
(3) No, I don't think that "patching" the furnace with anything is a good idea.
(4) Home CO detectors are good for higher CO levels. Low level CO can be an indicator of a major furnace problem. Low level CO (my MD friends tell me) may also produce various serious health problems which may imitate the flu, headaches etc.
(5) I urge you to get a new furnace.