I had my (1400s sq ft; 2 floor) house reviewed for energy efficiency recently. The guy stuck a probe in the flue of my oil boiler and gas water heater. He showed me Carbon Monoxide levels from the water heater was reading 10ppm and the boiler was 340ppm. He said the boiler was way over the acceptable level (100ppm) and I should call someone for a tune up. I have a service contract with my oil supplier and they come every year for "tune up". So I called them and that guy checked the flue venting with a meter and looked at the flame in the boiler. He told me that there was nothing wrong with the boiler and if there was too much CO I would see it as soot or black flakes in and around the boiler. He told me there's nothing wrong, not to worry, and left.
I don't know what to think now. The boiler is a Williams Oil-O-Matic and by it's name I'm guessing it's at least 30 years old. Is 340ppm an unreasonable amount of CO for a boiler of this age? Is there something wrong that needs fixing?