Where is the engine computer located and how is it reprogramed

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We are thinking we have low vacuum when running on 3 cylinders. We Have Replaced master cylinder and power assist unit, bleed brakes, 6 new coils & spark plugs and most maintenance due sensors Have searched for vacuum leaks and none to be found. Why When I take my foot off the gas pedal while traveling, the engine immediately drops from six cylinders to three cylinders. When I go to stop I barely have any breaks. I'm assuming that the reason for this is that when the engine drops down to three cylinders it does not provide enough vacuum for the breaks to work properly We are thinking reprograming computer Or the BIG sins of a DIY taking it to a dealer.