Was really thinking about car, but closed car still has air conditioning inlets, but I do not know if they provide oxygen when car is closed.
But there are situations when dog or a child is left in car in a really hot day and they die, but I assume it's due sweating and lack of water, not oxygen.
I believe when a dog or child is left in a car on a hot day they likely die from heat stroke and dehydration.
As for CO, it was my preferred method for a short while but I ultimately decided against it purely based on not having a car and not being able to light the coals, wait until they've finished burning and then transport them to whenever I need without being noticed or rousing some sort of suspicion. I definitely think when it's well prepared and done right it has a high rate of succession.
Some things I learnt while researching the method is a tent or a car is the best place to do it because it's a confined space so the affect works quicker but you need to be cautious about ventilation, particularly with a tent as they can have vents or a breathable material that could let the CO escape - so look at product specifics to know if it's suitable or not. Also with a tent or a car, there's less chance of it affecting anyone else. I had read a few cases where the car caught on fire from embers either before or after the person died so that's something to consider, too.
If you plan to do it at home, in a flat then there is the risk of CO leaking into your neighbours apartments. If you do plan do it in a sealed bathroom for example, it would be a good idea to open some windows in the rest of your flat so if any gas does leak out then it won't pose a risk to whoever enters your home and finds you.