Quote Originally Posted by amindzo View Post
but how can i calibrate the sensors in clean air? how can i understand that the value of potentiometer reaches to the suitable value ?
Most likely you cannot, Smog Test machines use calibrated, calibration gases just for that purpose, that contain various different mixtures, for certain CO is one of them, you might check your local Smog Test Station for near empty cylinders they might sell to you, or buy from an automotive parts distributor. Otherwise you will be on the hook to buy from a standards lab supplier.