After a parked car reaches a certain internal temperature with the keys off, a CO2 detector could detect an increase in CO2 levels and sound a siren. A heartbeat sensor could also be used, but might be more expensive.

Do not expect to replace common sense with a car system.

A child or dog doesn't make enough CO2 to increase the CO2 levels in the car. A person can sleep overnight in a car without suffocating.

Actually, the CO2 levels could be decreasing. When the car is driven on a road with lots of cars, CO2 made by the cars gets into the car and levels increase. Once it's parked in a parking lot other parked cars, with their engines off, the CO2 levels should start returning to normal, or at least closer to normal.

CO2 has nothing to do with that, the excessive heat is what can kill. The body will initially sweat trying to cool itself which will be ineffective without ventilation, then become dehydrated and feverish on its way towards heat stroke or worse. So what you would need to sense is the temperature and whether anyone was in the car.

CO2 is Carbon dioxide - has nothing to with kids

in left in hot car.an or speaker comes on

help there is child left in this car if it sense a child not moving

afyer say 2-3 mintues [or breathing]