Fixing a malfunctioning seat belt sensor

Maybe this will help you out seat belt sensor explanation

The sensor is only a hall effect sensor (reed sensor) it is base on magnetic reading. My guess is your magnet in the seat belt lid is wear off, try to use a strong magnet put it close to the buckle for tripping the sensor as always 'ON' that way your warning light will off.

If it doesn't go off then there is something wrong with your reed sensor, jumping the wire sensor is actually a solution too but you must open your buckle or try to locate the wire sensor.

This is cheapo way to turn off the warning light, I would not recommend this cheapo way if it is not necessary or emergency.

I took a look inside the female buckle in my car and noticed there was something that looked like an electrical contact inside, so I took a few q-tips soaked in hand disinfectant and scrubbed the inside of the buckle with those. You should be a bit careful not to get the cotton bud stuck in the mechanism if you try this. Another option would be to use a screwdriver and a cleaning wipe to clear the contacts.

Of course, as XC Source said in their answer, seat belt sensors are supposed to be magnetic, so this trick most likely won't help in a newer car.

This appears to have temporarily solved the problem. I ended up having to repeat the process a few times during a ten-hour ride, though.

After I got home, I booked a mechanic's appointment at a Ford dealership. I was informed that the buckle can't or won't be disassembled by a mechanic, so I'd be looking at a replacement. However, according to them, the important sensor is in the seat so we decided to cancel the appointment and keep an eye on the issue.