Bosch ZBR21-3A Error Codes

Updated October 13, 2026 · by Aurora Star Tech Inc.

Bosch ZBR21-3A error codes and diagnostic codes (also listed as ZBR 21-3A, ZBR 21-3 A). The ZBR21-3A is a Bosch boiler in the Bosch Greenstar series.

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Bosch ZBR21-3A error codes list

Code	Meaning	Severity
A7	DHW temperature sensor fault	Medium
A8	Communication fault	Medium
Ad	DHW tank temperature not detected	Medium
b1	Code plug not detected	Medium
b2/b3/b4/b5/b6	Internal data error	Medium
b7	Burner control fault	Medium
C6	Fan not running	High
CC	Outdoor temperature sensor not recognised	Low
d3	High limit / temperature limiter circuit open	High
d5	External supply temperature sensor fault (low-loss header)	Medium
E2	Supply temperature sensor fault	High
E3	Additional supply temperature sensor fault	Medium
E9	Safety high limit or flue gas limiter tripped	High
EA	No flame detected	High
F0	Internal fault	Medium
F1	Internal data error	Medium
F7	Flame detected with burner off	High
FA	Flame detected after gas shut off	Critical
Fd	Reset button pressed by mistake	Low

Troubleshooting the ZBR21-3A

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General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.

Bosch ZBR21-3A error codes list

Troubleshooting the ZBR21-3A

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