Bosch Thermotechnology · Error codes
Bosch Greenstar A8 — Communication Fault
A8
Communication fault. A communication fault has occurred, so check the BUS communications wiring and replace it if defective, and check the control.
Applies to
This communication or network fault code is displayed by the Greenstar control (fault codes on display) on Greenstar Combi models (100p = ZWB 28-3A, 151p = ZWB 42-3A), as documented in Greenstar Combi 100p/151p Installation and Operation Manual, doc 6721853953 (6721853953). Codes can differ between controller generations, so always check the manual that matches your unit.
- Bosch Thermotechnology Greenstar Combi (100p/151p)
- Bosch Greenstar Combi 100p (ZWB 28-3A)
- Bosch Greenstar Combi 151p (ZWB 42-3A)
Controller: Greenstar control (fault codes on display). Source manual: 6721853953 (2025-11).
Why a code is not a diagnosis
A communication code involves the link between controls (cascade, remote, sensors on a bus). Another unit being off, a wiring fault or an address conflict can all produce it.
Safe things to observe
- Check that every appliance in a multi-unit plant has power.
- Report the code together with any other codes on the system.
Do not
- Do not re-address or rewire control networks yourself.
How professional service helps
- A technician verifies power, wiring and settings across the connected units.
- Combustion and safety devices are verified with an analyser and the manufacturer's procedure after any repair.
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Frequently asked questions
Can I clear this code myself?
Clearing a code without fixing its cause usually just brings it back. Note the code, check the safe items on this page, then call a licensed technician. If your appliance manual describes a user reset for this exact code and it stays away, no call may be needed.
Boiler locked out? Call with the alert text ready.
The brand, model and this alert are passed along when you request service online, so the technician arrives prepared.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
- Greenstar Combi 100p/151p Installation and Operation Manual, doc 6721853953 · 6721853953 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.