Bosch Boiler: Lock the boiler at minimum or maximum output (function 9.A)
Lock the boiler at minimum or maximum output (function 9.A) on a Bosch boiler: Values: 00 normal, 01 permanently minimum output, 02 permanently maximum output. The display shows 9.A while active.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
How to fix lock the boiler at minimum or maximum output (function 9.a): step by step
- Reach the second level (service button, then ECO and key pad lock together) and scroll to 9.A
- Choose 01 or 02, and set 00 to return to normal
If the unit shows an error code, look it up: Bosch Boiler codes
Models
ZBR21-3AZBR28-3AZBR35-3AZBR42-3A
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. Written in our own words as a summary; always check the original manual.
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.