Chrysler Repair: Chrysler Repair Code P0457, chrysler pt cruiser, vacuum pump

Question
I have taken my 2005 Chrysler PT Cruiser to the dealer three times now for a P0457 Check Engine Light Code.  First time they charged $600 for replacing the Vacuum Pump and two Solenoids.  The second time they found a leak in a hose coupling (minimum charge).  The third time they replaced the fuel cap (no charge).  Now they want to replace the on-board computer for $3900.  I am thinking that the sensor closest to the gas tank is faulty.  The dealership says that is not the problem.  Please Help.

Answer
there is no sensor closest to the gas cap

P0457-LOOSE FUEL CAP


POSSIBLE CAUSES
GOOD TRIP EQUAL TO ZERO
LOOSE OR MISSING FUEL FILL CAP
INTERMITTENT CONDITION
NVLD SERVICE TEST
EVAPORATIVE EMISSION LEAK DETECTION
EVAP PURGE SOLENOID OPERATION
NVLD SWITCH OPERATION