is pinging dangerous? bought july 1. Pinging, eng lite on, guy took away a part

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The Spanish man who sold me the car took a plastic fitting that plugged into an alleged mass air flow sensor. Would not let me have it and said I could buy a junk one for $40. Metallic ping upon acceleration. Also a knocking noise intermittant. He and I did not know car needs to be smogged immediately. Mechanic could not diagnose or read codes for lack of the part (I guess). Now the guy wants to buy car back, desperately. He as going to smog it via "on of those places" whatever that means. Help? Is there more than I realize? Car feels hot, going thru gallons of fuel. Bought for $800 and all the money I have. Black smoke on exhaust pipe and bumper What to do? 310-940-2102