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2008 GX470 throwing code B1153 Seat Position Airbag Sensor Circuit Malfunction confirmed by a reputable independent shop and a Lexus dealership.
Independent shop states they took voltage measurements in dash and at various points under the seat, including the seat belt buckle covered in the 2003-2006 Technical Service Bulletin.
$2,400 for a new seat rack/rail (basically the seat frame and track with wiring), total $3,600 with labor.
$220 for a used seat rack/rail on eBay but the sensor could be bad in the used part.
Is it possible to replace just the sensor and not the entire seat rack/rail?
According to that TSB, it's either the seat belt buckle (with attached harness) or the "Floor Wire No. 2" harness are to blame.
Changing the seat belt buckle is a DIY - remove the seat and the buckle is right there. Part cost is about $70
I suspect that "Floor Wire No.2" is the interior body harness. You'll need to strip the interior and look at the part number on the label on the harness near the B pillar (there are many variations of it) to figure out what to order. Part cost is about $900. If you can get the wiring diagram from TIS you might be able to splice the damaged wire yourself and save a lot of money. But... it is a critical safety system. Up to you.
Unbolt and tilt up the driver's seat, and have a good inspection of the harness to the seat belt buckle for damages, it happened on mine and I was able to lengthen and repair the harness, it's been good now for about 9 years. The memory seating moving the driver's seat back and forth fatigues the wiring over time, leading to failures. $5 fix if you can find the damaged section.