To determine the concentration of carbon monoxide (CO) in the air with mathematical precision, it is necessary to use calibrated sensors and specific monitoring instruments, such as gas detectors. Lethal concentrations of CO are generally found between 1,000 and 10,000 PPM (parts per million). At 1,000 PPM, symptoms begin to appear within a short time, while at 10,000 PPM, exposure can cause irreversible health damage and death within minutes. These measurements must be done in safe environments, without exposing any individual to the risk of poisoning. The car runs fine, but you have to produce this monoxide inside the car. Maybe your car produces enough and you already know how to know if it's enough. Otherwise you will have to fill the car with some other method. When you have reached the right concentration, simulate entering the car by opening the door. Then close it and continue observing the detectors (from the outside). Check that you can maintain lethal concentrations for at least an hour.
With a four-hour exposure to 10,000 PPM of carbon monoxide, death is virtually guaranteed. At this concentration, asphyxia caused by the inability of the blood to carry oxygen becomes fatal relatively quickly. The longer the exposure period, the greater the likelihood that the body will fail to survive, so four hours at this concentration is far more than enough to cause death. Obviously it is not worth surviving because the damage is horrible for the body. Sometimes it is better to spend a few hundred euros more to do the tests, rather than venture into the unknown without exact data. Now you just need to know how to produce that concentration (10,000 ppm for at least an hour) or higher concentration while ensuring you don't get caught.