following a passenger seat base replacement at a Hyundai service center my 2009 sonata passenger seat belt light flashes while traveling at speeds over 10mph and does not stop until I slow to 6 MPH.... all while the seat is unoccupied...tech says that is what it is suppose to do.....didn't do it for 46,000 miles....any clues?????????? Because they don't have one.

 

Sounds like a problem with the weight sensor in the seat. The may not have connected the sensor correctly or at all. Or maybe the sensor is bad. Either way, the computer thinks someone is in the seat and that's why the seatbelt light is on.

Is the "Passenger Airbag Off" on or off? It should be on with nobody sitting in the seat but if it's not lit that tells me the computer thinks there is someone in the seat.

Check under teh seat for a loose wire/connector.

 

I agree. I own and have read the `09 Sonata (2 of 2) Shop Manual. Ref: RT Section of the Manual, page 12. All test to assure proper functionality of the restraint system is determine with the seat unoccupied/occupied. Tech refers to the owners manual where it says the tell tale lamp is lighted and flashes all the time but he doesn't take into consideration that the seat must be occupied. I explained that a "RED" warning tell tale lamp should never be flashing (without reason) during operation of a vehicle.
The Owners Manual isn't as detailed as the Shop Manual and he hasn't reviewed the Shop Manual for proper diagnosis of the problem. I realize the RT section I referenced is at the begining of the chapter under Air Bag operation but, it is important to understand all of the same switches and/or components apply to the seat belt restraint system as well.