2003 Ford Ranger Airbag code 21

I had a collision with my ranger which caused the airbags to deploy. Im trying to get the car fixed. I have replaced the Airbags (both sides) and module. I got the parts from a salvage yard. The airbag indicator light now gives me a code of 21. Could anyone tell me what this code means and how do i fix these problems. thanks

I solved the problem by replacing the seat belt pretensioners on both seats.