Is there any way of knowing if a seller cleared the codes for a check engine light with a scan tool, or disconnecting battery? Happens way too often.

There really i no way of knowing if codes have been cleared. If you get lucky and have a good scanner, (Modis, Autel, etc.) you may be able to locate history codes. If you are looking to purchase a vehicle, ask the seller to allow you to road test the vehicle. perform several key cycles and see if the check engine light comes on. Not guaranteed to work but it might help.

Thanks man!. This is such a dishonest practice. I heard that an emissions specialist has a way of knowing, like where you get it inspected. But yes, driving it a period of time(40 min) might uncover a scam but no guarantees.

During the road test, be sure to turn the car off and on a few times, that's the key cycling.

Thanks guys. Much appreciated