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 InspectionCleaning and protecting the vehicle interior
For the Subaru Solterra, interior cleaning should always be performed
according to the specific material of each component in order to preserve durability,
appearance, and long-term comfort.
Protecting the vehicle interior
In the Subaru Solterra, begin cleaning by removing dust and debris u ...
Crankshaft Position (CKP) Sensor Inspection [Mzr 2.0, Mzr 2.5]
Visual Inspection
CAUTION:
When foreign material such as an iron chip is on the CKP sensor, it can cause
abnormal output from the sensor because of flux turbulence and adversely affect
the engine control. Be sure there is no foreign material on the CKP sensor when
replacing.
...
If a warning light turns on or a warning buzzer sounds
If any warning indicator illuminates or a buzzer is activated in
your Subaru Solterra, remain calm and take appropriate action. A warning light that
briefly turns on and then goes off does not always indicate a malfunction. However,
if the issue repeats or persists, it is recommended to have t ...