Things used to be so easy when working in this business 'back in the day'. When carrying tools for some old timers, I remember some replacing an air filter and kicking the furnace while saying "Yup, still here!" Maintenance these days, for great reason, has taken a more efficient operation mentality, mainly because energy prices have started to affect our wallets. These days, 68% steady state efficiency oil boilers and gas furnaces with high carbon monoxide readings mean something. But, does it necessarily mean we must replace the furnace, boiler, hot water heater, etc.?
I heard a situation a while back where a representative of a local gas company, whether or not he was a subcontractor I cannot be sure, completed some maintenance on a gas boiler that he ended up shutting down due to a high CO reading. Most building analysts know that 26-100 ppm CO (in the flue, at steady state) means repair is necessary (see Combustion Safety Test Procedure, BPI.org), and to call a heating professional. So why did this tech shut down and tag this boiler for replacement? Was it actually a sales tactic?
High Carbon monoxide readings tend to point to what I like to call "unbalanced combustion". You see, it could be due to lack of combustion air, too much gas, or too much excess air. No matter how you slice it, the combustion process is out of balance. I have seen high CO readings for draft inducers failing, secondary heat exchanger condensate drains clogging, improper manifold gas pressure, incorrectly adjusted draft dampers, you get the idea! None of these reasons mean the equipment has to be replaced, but the recommendation could be made.
The boiler I started talking about earlier had a manifold gas pressure twice the recommended value. There was not enough combustion air due to a lack of understanding and/or measurement. Since he took the readings, I doubt it was the latter. Looking at the flame on a 90%+ AFUE appliance will never be a justified reason to crank up the manifold gas pressure. If you cannot measure with a combustion analyzer, or even gas pressure with a electronic manometer, at least clock the gas meter before you start creating high CO2 and CO outputs. You are the professional and the homeowner is paying you for the expertise. By all means, please, don't recommend replacing a boiler your company may have installed just a a couple years ago!