Bosch Error Code 6.A: Service function 6.A – latest fault code (Boiler)
Bosch error code 6.A on a boiler means: Service function 6.A - latest fault code. It is also called Bosch diagnostic code 6.A.
Homeowner? What Bosch code 6.A means for you
Usually minor Your boiler is reporting: Service function 6.A - latest fault code. This code is usually informational or minor. The safe checks below often clear it; call us if it keeps showing up.
Safe things you can check yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the boiler gas valve is open (handle in line with the pipe).
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Bosch error code 6.A mean?
Shows the most recent fault code stored in the boiler. Read-only.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
Bosch models that use code 6.A
ZBR21-3AZBR28-3AZBR35-3AZBR42-3A
Other Bosch Boiler codes
All Bosch Boiler error codes →
Bosch code 6.A: homeowner questions
What does Bosch code 6.A mean?
Service function 6.A - latest fault code. Shows the most recent fault code stored in the boiler. Read-only.
Is it safe to keep using my boiler with code 6.A?
Usually yes. Code 6.A is normally minor, but if it keeps coming back have it checked.
Can I reset Bosch code 6.A myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code 6.A comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code 6.A near me?
Aurora Star Tech Inc. repairs Bosch boilers in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
Homeowner help: Boiler Not Heating: What to Check
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
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.