There is a significant difference between CO2 (carbon Dioxide) and CO (carbon Monoxide). While CO2 in sufficient quantities can displace oxygen in an enclosed space, it's not really considered immediately harmful to humans otherwise. So don't discharge your CO2 extinguisher inside an enclosed cab.
CO, on the other hand, is poisonous when measured in parts per million and explosive when measured in percentages. It has a high affinity for the hemoglobin in your blood, preventing oxygen from reaching your brain, heart and other tissues. It is colourless and odourless but is a by-produce of any fuel-burning device, so if you smell exhaust then you are inhaling CO too. Symptoms are "flu like": headache, nausea, dizziness, drowsiness and confusion. Followed by weakness, sleepiness and death.
The harm caused by CO is based on the time and exposure level. 9ppm or less is not a concern. 10 and over is considered unacceptable in the home, where people often spend more than 8 hours inside (especially at night) and children or pregnant women may be present.
In a work environment, a level of 25ppm is usually considered acceptable to a maximum of 8 hours but that quickly ramps to the point where 100ppm is considered immediately dangerous. For a healthy adult of working age: Limits for the elderly, pregnant women and children are half that. Long term exposure to low levels can be a serious risk for those with heart disease, angina, clogged arteries, etc..
Inexpensive battery-powered CO alarms intended for installation in the home will not alarm below 30ppm. They must alarm in 60 to 240 minutes at 70ppm, 10 to 50 minutes at 150ppm and 4 to 15 minutes at 400ppm.
When new.
They use cheap electro-chemical sensors the become desensitized when exposed to low levels of CO over a long period of time. In other words, they may not work if they are constantly subjected to low levels over time. It is recommended that they NOT be installed near sources of occasional puffs of CO, like stoves, furnaces and engines for that reason.
I strongly suggest that you should not rely on a household CO alarm to protect you inside the cab of your tractor.