CAPT OF A B737-300 FOUND THE CTL DISPLAY UNIT (CDU) SET TO THE WRONG ENG. PROB CAUSED BY A SWITCH USED BY MAINT WHEN A SLIGHTLY DIFFERENT MODEL ENG IS INSTALLED.

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IN THE COURSE OF COMPLETING THE NORMAL PREFLT OF OUR B737-300; AS WELL AS THE REQUIRED PAPERWORK; OUR RELEASE AND ALL OTHER PAPERWORK INDICATED THAT THIS ACFT WAS EQUIPPED WITH CFM26-B2 ENGS. DURING THE COCKPIT SETUP; INFO DISPLAYED IN THE CTL DISPLAY UNIT INDICATED THAT THE SOFTWARE WAS FOR B1-PWRED ENGS. GIVEN THE PREPONDERANCE OF DATA WHICH TOLD US TO EXPECT B2 ENGS; THIS ONE ENTRY ON THE COMPUTER DISPLAY COULD EASILY BE MISSED BY CREWS OPERATING THIS ACFT. AS I ASSUME IT WAS BY THE CREW WHO BROUGHT THIS ACFT TO MEX. OUR MAINT DEPT INDICATES THAT SOME PORTION OF OUR FLEET HAS BEEN EQUIPPED WITH A SWITCH; WHICH THEY CAN CHANGE FROM ONE ENG RATING TO THE OTHER WITHOUT HAVING TO COMPLETE EXTENSIVE; TIME CONSUMING PROCS SHOULD A DERATING OR ENG REPLACEMENT BE NECESSARY. THEIR SUPPOSITION IN THIS CASE IS THAT THIS SWITCH MIGHT HAVE BEEN INADVERTENTLY BUMPED AND SWITCHED TO THE INCORRECT POS. THIS SIT HIGHLIGHTS THE PROBS FACED BY TODAY'S AIRLINE CREWS UTILIZING ADVANCED TECHNOLOGY EQUIP WHICH PROVIDES ACCESS TO ALMOST UNLIMITED INFO; BUT CAN; ON OCCASION; PROVIDE INCORRECT DATA WHICH AFFECTS SAFETY OF FLT. THE PERCEIVED INFALLIBILITY OF THIS EQUIP (COMPUTERS) LEADS TO A COMPLACENCY WHICH COULD BE FATAL.