Possible Causes

WARNING: This page is about a different car, the 2009 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

Driver-side pre-tensioner seat belt connector malfunction
Open or short or short to ground or short to power supply circuit in the wiring harness between the driver-side pre-tensioner seat belt and SAS control module
Driver-side pre-tensioner seat belt malfunction
SAS control module malfunction