Bosch Thermotechnology · Error codes
Bosch Greenstar d3 — Temperature High Limit Defective; External Guard Tripped; Temperature Limiter Locked Out
d3
Temperature high limit defective; external guard tripped; temperature limiter locked out. The temperature high limit is defective, an external guard has tripped, or the temperature limiter is locked out, so check the temperature sensor, the leads, and the jumper across 8-9 or PR-P0.
Applies to
This overheating code is displayed by the Greenstar control (fault codes on display) on Greenstar FS floor-standing models (KBR/KWB 21–42-3A), as documented in Greenstar FS Installation and Operation Manual, doc 6720810590 (6720810590). Codes can differ between controller generations, so always check the manual that matches your unit.
- Bosch Thermotechnology Greenstar FS (floor-standing)
- Bosch Greenstar KBR 21-3A
- Bosch Greenstar KBR 28-3A
- Bosch Greenstar KBR 35-3A
- Bosch Greenstar KBR 42-3A
- Bosch Greenstar KWB 28-3A
- Bosch Greenstar KWB 35-3A
- Bosch Greenstar KWB 42-3A
Controller: Greenstar control (fault codes on display). Source manual: 6720810590 (2025-11).
Why a code is not a diagnosis
An over-temperature code protects the heat exchanger or venting; it does not reveal whether the cause is low water flow, a failed pump, trapped air, fouling or a sensor fault.
Safe things to observe
- Note the code and whether rooms or hot water are still working.
- If you see steam, melted wiring or smell burning, switch the appliance off at its service switch and call a technician.
Do not
- Do not reset the appliance repeatedly while the cause is unknown.
- Do not remove covers or bypass any high-limit device.
How professional service helps
- A technician checks pump operation, flow, system pressure and the cleanliness of the heat exchanger.
- 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.
Parts and service
Parts commonly involved
These parts appear in the documented text for this alert. A code reports a condition, not a failed part — a technician confirms the cause by testing before anything is replaced.
High-limit switch
A high-limit switch shuts the burner off when boiler water gets hotter than it should, protecting the heat exchanger and the system from overheating.
What a technician checks →Boiler sensors (thermistors)
Boiler sensors are small temperature probes, usually thermistors, that tell the control how hot the water and flue gases are so it can fire safely…
What a technician checks →
Service for this alert
- Gas Boiler Repair
Licensed gas technicians from ROMA Heating & Cooling diagnose and repair residential and light-commercial gas boilers across…
Every boiler part explained — what it does, how it fails and what a technician checks.
Documented for Bosch Thermotechnology Greenstar FS (floor-standing).
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 FS Installation and Operation Manual, doc 6720810590 · 6720810590 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.