4 Wheel Drive Transmission Problem

My 2005 4 x 4 auto Pathfinder (which I purchased from new) sometimes engages 4 wheel drive without being selected and will not engage when it IS selected.  It has been back to the main dealer 3 times.  All fuses etc checked, 2 new switches and throttle control unit fitted (as per computer diagnosis) - great expense but no improvement!  Computer shows fault on fuel pump, what is the connection? Main Dealer suggests trial and error elimination.  Nissan will not refund unused spares to the Main Dealer who is reluctant to order expensive spares and therefore will not stock them, keep the vehicle for a couple of days and 'hopefully' eventually solve the problem.  Instead, I am required to make return visits in order that the cheaper spares can be fitted and the fault found before more expensive parts may be necessary.  The problem is - it is expensive and most inconvenient to make return visits and the final cost could be prohibitive and for example if the 12th spare cures the problem the other 11 would have been fitted in error.  The dealer will understandably say all the faults showed up on the computer but I cannot understand how most or all of them affect the original problem.  Your help and suggestions would be very much appreciated.

Hi aylnissan,

Thanks for your time.--FWD actually engages, by itself in low, max' speed 10 mph, light on dash'. Fortunately ??!! it only happens when shifting from neutral to drive. When I want FWD ,the axle symbol changes but the light ( FWD ) is not on and 4 x 4 is NOT engaged.

Kind regards,

Shackleman