Bosch Boiler: Open the service functions (Greenstar service button)
Open the service functions (Greenstar service button) on a Bosch boiler: Holding the service button until it lights opens the service functions; the display shows a function number such as 1.A. Second-level functions (8.A onward) need ECO + key pad lock together. Service mode times out after 15 minutes without a button press.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
How to fix open the service functions (greenstar service button): step by step
- Keep the service button pressed until it lights
- Scroll with the key pad lock button (down) or ECO button (up)
- Tap the emissions test button to see a value; change it with the same two buttons
- Hold the emissions test button to save, or press it briefly to leave without saving
- Second level: with the service button lit, press ECO and key pad lock together until 8.A shows
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), 11.1 Guideline to service functions p.57-58. 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.