Before you answer with “Duh, of course it’s not!”, read my reasoning:
Burning something is really just reacting that thing with oxygen, causing an exothermic chain reaction.
CO2 is CO with an extra O atom.
I am presuming it must be possible to produce CO2 from CO and O2.
If I combine 2 parts CO and 1 part O2 and introduce a spark, will it ignite?
It is possible to create CO2 from CO +O2. Your car’s catalytic converter does it every time you drive.
The reaction is not what I would consider self sustaining as it requires a catalyst and a very high temp to occur (>540F).
To the best of my knowledge the reaction is not exothermic.
Yes carbon dioxide is flammable. Its EU hazard rating is “very flammable.” It is flammable when it is between 12.5% and 74% of the gas mixture.
It’s even a component of wood gas which was used as a substitute for gasoline during WW2.
Umm what? Carbon dioxide is flammable? Why do they put it in fire extinguishers then?
Did you mean monoxide?
Yes CO is flammable and of course it’s an exothermic reaction - ever heard of coal gas or town gas? It puts out about 320 Btu/scf, which is roughly 1/3 of natural gas. Entire cities used to use it for lighting and cooking and even home heating, and I have personally worked on projects where power plants have co-fired it with coal or some other fuel to produce power. The ASME PTC 4.1 lists CO production as an efficiency loss, meaning that it’s carrying away useful energy by not completing the oxidation of C to CO2.
CO is even an explosion hazard, and you may not even need a spark. I have personally been to a site where a CO explosion killed a plant worker when he opened an inspection door at the wrong time and fresh air rushed in to combine with 400 F+ CO which had accidentally formed. I helped investigate the explosion incident and while it was possible a spark may have formed from static discharge, our investigation could not find any proof of that, and lab tests showed that it was likely a patch/hot spot of glowing unburned coal led to the explosion.
In addition to wood gas, producer gas is mostly CO. Prior to the rise in use of natural gas, most town gas supplies were created by destructive distillation of coal or various exothermic reactions involving coal or coke. Coal gas contains CO. Producer gas was the term used for the reaction that made lots of CO from coal. Water gas added steam to the process and yielded mixes including hydrogen and methane. Syngas is more advanced, and also yields CO in the mix. The town gas supply could in principle be made form a mix of sources, but was usually Syngas. Critically, the CO content of town gas was always significant. This is why people were able to commit suicide by sicking their head in an unlit gas oven. CO is not a great fuel, it is is after all essentially already half burnt. But it was easy to make, and as a gas, had all the advantages of easy distribution that makes gas so useful as a fuel.
Pah, ningaed, 
So why does CO2 work as a fire retardant? Virtually every commercial fire extinguisher uses pressurized carbon dioxide as its main ingredient.
Because while carbon monoxide is flammable, carbon dioxide is not.
And burning CO forms CO[sub]2[/sub].
Bolding / Size by me
My guess is BigT meant to say Monoxide and just hasn’t been back here to see his error. His link references a report which includes info on the flammability of Carbon Monoxide.
The next person who confuses CO and CO2 in this thread will be eaten by a Stegosaurus stenops. And yes, contrary to what the Bible tells us, they really are carnivores. And they’re hungry.
Note that when you distil coal you get some volatile combustible fractions just like you do with petroleum, and some heavier tarry fractions as well as the coke.
Water gas gives you methane and carbon monoxide, producer gas just CO and nitrogen (assuming you’re blasting air), so producer gas is poor fuel but the reaction is exothermic, whereas the water gas reaction is seriously endothermic (but makes better fuel). So by periodically blasting air to make producer gas, you can heat the coke enough for another water-gas run, and so on.
CO can be seen burning in a charcoal or coke brazier - at the heart of the fire the glowing coals aren’t getting enough air for complete combustion, so you get some blue gas-flames at the top where the rising CO reaches the fresh air and burns the rest of the way.
I think there is only one of that is possibly confused. The rest of us are commenting on post #3.
Ah, but since CO burns to form CO2, is it self extinguishing?
<runs>
Why do you tempt the stegosaurs?
And you…do you know how many pounds of grade-A prime Doper a stegosaurus needs to eat each day? Because if you do, please tell me…
Dihydrogen monoxide is also an excellent fire retardant, although extremely hazardous.
Stegosaurs are surprisingly docile after a delicious meal of doperflesh. Just don’t try riding them; they’re pointy.
As for CO self-extinguishing, I’m sure it’s only flammable (or explosive) under certain mixtures and temperatures, but I imagine the limiting factor in most cases is going to be the fuel running out.
Oh and there are some sorts of fires you shouldn’t put out with carbon dioxide (or water), but most of you probably don’t do anything that requires a class D extinguisher.
Getting back to the OP, I recall that when I was a kid in Philly in the 40s, we used something called water gas which I thought was a mixture of CO and CH4 and was extremely poisonous. Then one day after the war, the gas company came around and changed the burners (or at least the aperture) on our stoves and after that we burned natural gas. Although there is a danger of explosion, at least it wasn’t dangerous. I have wondered how the transition was handled. Some people in every neighborhood would be away for a day, whatever. How could they change everybody at the same time and what were the consequences of using natural gas with a stove tuned for water gas?
This was a problem for the Windscale nuclear fire - the aircooled pile was ablaze, burning graphite, uranium and other metals (the pile was used to irradiate various isotopes and produce plutonium). They tried CO[sub]2[/sub], but the burning metals stripped O from the CO[sub]2[/sub] and kept burning. H[sub]2[/sub]O was even more risky, as the O gets stripped out and you are left with H[sub]2[/sub], ready to boom. This didn’t help, and so the operators sealed the air intakes and cooling vents to starve the fire, which succeeded.
Si