If the Garmin bike computer detects an error with the data field or sensors, error codes are shown on the display.
B1" means bike 1, "B2" for bike 2, etc.
The number after the colon is the error code with the following meaning
:1 The Airspy ID of the front wheel is faulty or missing.
:2 The Airspy ID of the rear wheel is incorrect or missing.
:3 No Airspy ID entered or incorrect.
:4 The same ID has been entered twice.