Bosch Boiler Error Codes

Updated October 15, 2026 · by Aurora Star Tech Inc.

50 Bosch Boiler error codes and diagnostic codes, with causes and troubleshooting steps for each. Select a code, or find your model below.

Homeowner with a boiler problem? Call (647) 657-2977 or book a service call — licensed technicians across Vaughan and the GTA.

Bosch Boiler error codes list

Code	Meaning	Severity
000-199	Supply temperature readout	Low
2.C	Service function 2.C - purging function	Low
2.F	Service function 2.F - temporary test operating mode	Low
4.E	Service function 4.E - appliance type detected	Low
4.F	Service function 4.F - trap filling routine	Low
5.A	Service function 5.A - reset service-due indication	Low
6.A	Service function 6.A - latest fault code	Low
6.b	Service function 6.b - room control voltage at terminal 2	Low
6.C	Service function 6.C - target supply temperature from outdoor reset	Low
6.d	Service function 6.d - DHW turbine flow rate	Low
7.A	Service function 7.A - burner and fault indicator lamp	Low
7.b	Service function 7.b - 3-way valve to center position	Low
7.E	Service function 7.E - building drying function	Low
8.A	Service function 8.A - software version	Low
8.b	Service function 8.b - code plug number	Low
8.C	Service function 8.C - gas burner control (GFA) status	Low
8.d	Service function 8.d - gas burner control (GFA) fault	Low
8.E	Service function 8.E - restore factory settings	Low
8.F	Service function 8.F - permanent ignition test	Low
9.A	Service function 9.A - constant operating mode	Low
9.b	Service function 9.b - current fan speed	Low
9.C	Service function 9.C - current boiler output	Low
A.b	Service function A.b - DHW temperature	Low
A.C	Service function A.C - DHW tank temperature	Low
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
C.d	Service function C.d - current heat demand	Low
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
DHW setpoint flashing	Set DHW temperature flashing on display	Low
Display off	Display dark - appliance switched off	Low
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
ECO button lit	ECO button lit - economy / alternating mode	Low
Emissions test button	Emissions test (chimney sweep) button	Low
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
Key pad lock button	Key pad lock	Low
Service button	Service button - open service functions	Low

Find codes by Bosch model

ZBR21-3A ZBR28-3A ZBR35-3A ZBR42-3A

Boiler troubleshooting guides

No heat or some zones cold All brands · Boiler
Force minimum or maximum output for 15 minutes (function 2.F) Bosch · Boiler
Lock or unlock the key pad Bosch · Boiler
Lock the boiler at minimum or maximum output (function 9.A) Bosch · Boiler
Move the 3-way valve to centre to drain (function 7.b) Bosch · Boiler
Open the service functions (Greenstar service button) Bosch · Boiler
Read the latest stored fault code (function 6.A) Bosch · Boiler
Reset the service-due indication (function 5.A) Bosch · Boiler
Restore factory settings (function 8.E) Bosch · Boiler
Start or stop the building drying program (function 7.E) Bosch · Boiler
Start the air-purge routine (function 2.C) Bosch · Boiler
Start the emissions test (chimney sweep) mode Bosch · Boiler
Test ignition with continuous sparking (function 8.F) Bosch · Boiler

Homeowner help: Boiler Not Heating: What to Check

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 Boiler error codes list

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Boiler troubleshooting guides

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