I took my car in for a tune up to a friend, he changed the spark plugs and wires, changed the timing belt and water pump, and then the car would drive great on the road but would die out at the stop signs. It starts right back up, but will die as soon as you take your foot of the gas. He then said it needed the O2 senors replaced so we replaced all four of them and now he says he thinks it needs mas air flow senor bcause of the codes that come on when he puts in on the diagnostic machine. It says it can't read any it. I think the timing is off but he said he checked it again. What can it be?