I’m considering writing a story in which the main character tries to kill his family with the CO from an oil furnace. Does anyone know if blocking all the external air vents to the furnace would lead to improper combustion capable of producing lethal levels of CO?
That sounds right, though wouldn’t a tampered vent be discovered?
Yes, the tampered vents would definitely be discovered, but the protagonist won’t be thinking that far ahead. All he’ll be concerned with is killing his family in a non-violent, fairly passive way, for which carbon monoxide poisoning works well. He has no plans to elude detection or capture.
If you’re talking about blocking the exterior outlet vent for the burner, then the answer is yes it could, but not necessarily.
Blocking the combustion outlet vent would cause the products of combustion to “back up” through the burner, and depending on the design of the burner, it could then easily spill out into the heated air space, where it could spread through the home. This is exactly the kind of thing you’re trying to protect yourself against by putting CO monitors in your home.
The typical home furnace (especially older models) typically produces some CO along with CO2, water, and other miscellaneous stuff. Additionally, as you mention, blocking the easy air flow through the burner will likely cause the burn to be sub-optimal, and may well increase the amount of CO produced.
But some designs have sealed combustion chambers, or separate combustion air intake which is isolated from the heated air, or flow sensors (usually a vane switch or pressure switch) which automatically shut off the burner if the air flow is stopped. Any one of these designs would stop the CO poisoning unless steps were taken to bypass them.
In the interest of not spreading unfortunate tips for aspiring mass-murderers 
, I won’t elaborate on the bypass methods except to say that some designs would be pretty tricky to get around, while some would be pretty simple. If you’re interested, email me and I can go into more detail.
Ugly
You could have the bad guy sabotage the furnace so that it does not work and have him bring in a charcoal grill to keep the house warm at night. Plenty of CO, and he could claim stupidity (until the tampered furnace is found out.)
Wonder - why MODs don’t close this thread. Next someone will be writing a story about killing passengers on a flight …