Zero error code 21
Brake Bulb Out Fault
What it means
Brake Bulb Out Fault
Brake light connection open - bulb may be out.
Warning — Monitor the situation, may self-resolve.
What to do
- 1Check/replace brake light bulb or connection.
Code 21 on the other dash
The number alone is not enough. A Gen3 dash reads code 21 as Brake Bulb Out Fault; a Gen2 dash reads the same number as Board Vpack Error. And the model name does not settle it: the S, SR, DS and DSR kept their names when they moved from the Sevcon dash to Cypher III in 2024, so it is the year that tells you which table applies.
Which bikes use this table
Cypher III dash: SR/F and SR/S (2020+), DSR/X (2023+), S/F, and the S, SR, DS and DSR from 2024, when those four moved to the FST platform.
Related codes
Other Gen3 Lighting codes:
Questions
What does error code 21 mean on a Zero Gen3 dash?
Brake Bulb Out Fault. Brake light connection open - bulb may be out. It is classed as warning: monitor the situation, may self-resolve.
What should I do about it?
Check/replace brake light bulb or connection.
Does code 21 mean the same thing on a Gen2 Zero?
No. On a Gen2 dash, code 21 is "Board Vpack Error" — a different fault entirely. The two tables share numbers but not meanings, so read the one that matches your dash, not your model name: the S, SR, DS and DSR kept their names when they moved to Cypher III in 2024.
See when this actually happened on your bike
A dash code tells you the fault; the log tells you the pack temperature, the state of charge and the ten minutes leading up to it.
Sources:
The code and its name are transcribed from Zero's own owner manuals; the severity, category and troubleshooting notes are ours. For official diagnostics, consult an authorized Zero Motorcycles dealer.