1. Disconnect the brake fluid level sensor connector from the master cylinder.
2. Inspect for continuity according to fluid level between the brake fluid level sensor terminals.

If not as indicated in the table, replace the reserve tank.

Brake Switch InspectionHow to use AUX mode (Type B)
Select the icon on the home screen to display the Entertainment screen.
Select to switch to the AUX mode. The following icons are
displayed in the lower
part of the center display.
Icon
Function
Displays the Entertainment menu. Use to switch to a diffe ...
Operation
System triggering conditions
The horn sounds intermittently and the
hazard warning lights flash for about 30
seconds when the system is triggered by
any one of the following:
Unlocking a door with the key, door
lock switch, or an inside door-lock knob.
Forcing open a door, the hood or th ...
Contact Authorized Mazda Dealer and Have Vehicle Inspected
If any of the following warning lights or the indicator light turns on/flashes, the system may
have a malfunction. Contact an Authorized Mazda Dealer to have your vehicle
inspected.
...