there is a code you can do to disable that... it involves the seatbelt, the door, and putting the truck in drive, reverse, and neutral in some sort of combo. My dads '06 3500 did the same thing and a ex cummins certified mechanic friend of ours turned it off that way. I will try to find out how he did it. and let ya'll know!
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2001, 2500 5.9 H.O. 6 spd. SWB, 4x4,4.10 gears, South Bend Performance Clutch, FASS 150/150, FASS Draw Straw, 5'' turbo back straight pipe, Edge w/attitude, 75hp DDP Injectors, S&B cold air intake, Banks Ram Air Intake, Silencer ring MIA