HVACopediaBosch · Boiler
8.b Service function 8.b - code plug number
What the display is showing
Displays the final four digits of the code plug number. The code plug sets the appliance functions and must be replaced when converting between natural gas and LPG. Read-only.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
What to do
- Press the service button until it lights
- Press ECO and key pad lock together to reach the second level
- Scroll to 8.b
Models that show this code
4 models in Bosch Greenstar
Other codes on the same units
- 8.A Service function 8.A - software version
- 8.C Service function 8.C - gas burner control (GFA) status
- 8.E Service function 8.E - restore factory settings
- 8.F Service function 8.F - permanent ignition test
- 8.d Service function 8.d - gas burner control (GFA) fault
- 9.A Service function 9.A - constant operating mode
- 9.C Service function 9.C - current boiler output
- 9.b Service function 9.b - current fan speed
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Source: Bosch Greenstar wall ZBR-ZWB 21-42-3A service 6721853955 (2025-11) (same content in combi service 6721853953), 11.3 Description of the service functions pp.59-64. Summarized in our own words; check the original manual for your model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.