Depress the button on the seat belt buckle.
If the belt does not fully retract, pull it out and check for kinks or twists. Then make sure it remains untwisted as it retracts.

NOTE If a belt does not fully retract, inspect it for kinks and twists. If it is still not retracting properly, have it inspected at an Authorized Mazda Dealer.
Fastening the Seat Belt
Front Shoulder Belt AdjusterAir Bag Module And Pre Tensioner Seat Belt Deployment Procedures [Two Step Deployment
Control System]
WARNING:
A live (undeployed) air bag module or pre-tensioner seat belt may accidentally
operate (deploy) when it is disposed of and cause serious injury. Do not dispose
of a live (undeployed) air bag module and pre-tensioner seat belt. If the SSTs
(Deployment tool and Adapter harnes ...
On Board Diagnostic System PID/Data Monitor Inspection [FS5 A EL]
NOTE:
The PID data screen function is used for monitoring the calculated value
of input/output signals in the module. Therefore, if the monitored value of
the output parts is not within the specification, it is necessary to inspect
the monitored value of input parts corresponding to ...
Check Valve Inspection [Skyactiv G 2.0]
Airflow Inspection
1. Remove the vacuum hose..
2. Blow air into the vacuum hose using your mouth from the vacuum pump side and
verify that the air flows to the intake manifold side, then blow air from the intake
manifold side and verify that the air does not flow to the vacuum pump side.
...