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Chrysler did a wonderful thing when they started to put computers into their cars back in 1984, they enabled a feature where a technician could call up a stored computer fault code easily and display it's number on the odometer of the vehicle. This eliminated the need to drag out an (at the time) big and bulky machines to read the codes. This only displayed the code, however, it did not allow the tech to actually repair it from here. DIY Mechanics quickly learned of this and have used it to avoid going to the dealer every time the check engine light came on.