Bosch Thermotechnology · Error codes
Bosch/Buderus GB312 4A 520 — Boiler Safety Temperature Limiter (stb).
4A 520
Boiler safety temperature limiter (STB). The supply temperature has reached 200 °F (93 °C) (cut-out temperature of boiler temperature limiter). Checks: Fault can only occur as a result of faults in the system configuration (e.g. only in 2-boiler systems). Remedy: Check system configuration. Check orientation of check valve (direction of flow). The table classifies it as V (interlocking).
Applies to
This communication or network fault code is displayed by the Logamatic BC10 basic control unit and MC10 control unit (SAFe fault codes) on Logano plus GB312 models (90–280), as documented in Buderus Logano plus GB312 Installation and Service Instructions, doc 6 720 806 273 (01.2013), section 10.2.2 Table 23 Fault displays (6 720 806 273). Codes can differ between controller generations, so always check the manual that matches your unit.
- Bosch Thermotechnology Buderus GB312 (Logano plus GB312)
- Buderus GB312/90
- Buderus GB312/120
- Buderus GB312/160
- Buderus GB312/200
- Buderus GB312/240
- Buderus GB312/280
Controller: Logamatic BC10 basic control unit and MC10 control unit (SAFe fault codes). Source manual: 6 720 806 273 (2013-01).
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
- Buderus Logano plus GB312 Installation and Service Instructions, doc 6 720 806 273 (01.2013), section 10.2.2 Table 23 Fault displays · 6 720 806 273 · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.