My fuel gauge doesn't read properly, sorry... it'* a 2003 lesabre. Anyway, the gauge only reads correctly when the igintion switch is turned on w/o starting the engine when started it jumps to anywhere from 3/4 to full when I know it'* about a half a tank of fuel, also I have to engage my highbeam switch on the column to start the car... is this a common feature on Buick lesabres ? Someone told me it was an anti theft feature.

Sounds like you have a bad ignition switch

If it is an antitheft feature, it'* definitely aftermarket.

Hopefully the person that told you that was the previous owner, or someone who knew about this specifically on this particular car.

As for the fuel gage, you probably need a new fuel sender......