DTC 39 Seat Belt Warning Failure: DTC Detecting Condition

WARNING: This page is about a different car, the 2013 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.

Airbag control module is faulty.
Body integrated unit is faulty.
Harness circuits between body integrated unit and airbag control module are open, shorted or shorted to ground.

CAUTION:

Before diagnosing the airbag system, be sure to turn the ignition switch to OFF, disconnect the ground cable from battery, and wait 60 seconds or more before starting to work.
When replacing the airbag module, seat belt pretensioner, roll connector, control module and sensor, reconnect each part and check that the warning light operates properly.
When inspecting the airbag main harness, disconnect the airbag module connectors of the driver's and passenger's seats for safety.