The sensor panel hinges can SNAP OFF during removal. Old plastic is brittle. Plasticizer migration. (I offer a $35 solution since thin epoxy glue will not hold for long. My contact details are in my profile.)
Two weeks ago, I saw another broken hinge during a complex stairlift repair of your vintage. The breakdowns were inadvertently caused by the invalid’s husband. He said he “tinkered” with the 120 before I was contacted. Those folks already spent $2K so far. They would have saved a lot of trouble if they took up my $150 pre-purchase stairlift inspection service, which I perform, worldwide. Due to their budget, the husband said he would finish the repairs that I listed for him, with fix details. Unfortunately, two of those items are critical SAFETY components, which put his wife at risk of ACCIDENTAL DEATH.