My first thought would be something broken on the cassette itself. HPs aren't known for bad sensors. If you have to bite the bullet the whole options feed assy isn't that expensive...
HP M601/M602/M603 Series 500 Sheet Feeder With Tray
used on ebay only 50 bucks
HP M600/M601/M602/M603 500-SHEET PAPER TRAY FEEDER R73-0016 CE998A ~ FREE SHIP! | eBay
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