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 InspectionEngine Exhaust (Carbon monoxide)
WARNING
Do not drive your vehicle if you smell exhaust gas inside the
vehicle:
Engine exhaust gas is dangerous. This gas contains carbon monoxide (CO),
which is
colorless, odorless, and poisonous. When inhaled, it can cause loss of
consciousness and
death. If you smell ...
Stop Vehicle in Safe Place Immediately
If any of the following warning lights turns on, the system may have a
malfunction. Stop the
vehicle in a safe place immediately and contact an Authorized Mazda Dealer.
...
Steering Switch Inspection
Without Bluetooth System
WARNING:
Handling the air bag module improperly can accidentally deploy the air bag
module, which may seriously injure you. Read the air bag system service warnings
and cautions before handling the air bag module..
1. Disconnect the negative battery cabl ...