Engine Hours Changed On Dash Diplay

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I recently obtained a 06 2500HD 6.0 slt sierra CC 4X4 and engine hrs was at 827 had it 4 weeks now and it changed to 0. I'm not ref. to the trip timer. Should I be suspect of tampering ? I also should mention that the truck has low miles for the amount of years. Was first sold 09-24-05 had 27k after 3yrs. the mileage has not changed at all. Was a single owner & cert. preowned at the dealer when I bought it. I have also noticed that the passenger door light will stay on while driving, could it be a electrical issue with this truck? The light will turn off if I flip the cab lights switch on off. This light problem is intermitent. Maybe thats why I only paid 18k. Truck bluebooks at 26k retail &21k wholesale. thanks for the help in advance