Viessmann Vitocrossal 300 F9 — Burner in Fault Mode
F9
Burner in fault mode. The fan speed is too low during burner start, so the burner is in fault mode; check the fan, the fan cables, the power supply and the fan control, and press reset button R. The table classifies it as Burner in fault mode.
Applies to
This venting or air-supply fault code is displayed by the Vitotronic 200 control (CU3A) on Vitocrossal 300 CU3A models (CU3A-94, CU3A-125, CU3A-160, CU3A-199), as documented in Vitocrossal 300 CU3A Service Instructions, doc 5673 649 - 20, 03/2025 (5673 649 - 20). Codes can differ between controller generations, so always check the manual that matches your unit.
- Viessmann Vitocrossal 300 (CU3A)
- Viessmann Vitocrossal 300 CU3A-94
- Viessmann Vitocrossal 300 CU3A-125
- Viessmann Vitocrossal 300 CU3A-160
- Viessmann Vitocrossal 300 CU3A-199
Controller: Vitotronic 200 control (CU3A). Source manual: 5673 649 - 20 (2025-03).
Why a code is not a diagnosis
A venting or air-supply code points at the combustion-air path: a blocked terminal, a restricted flue, a failed fan, or a condensate restriction. It does not say which, and running an appliance with a blocked flue is unsafe.
Safe things to observe
- If a carbon-monoxide alarm sounds anywhere in the building, leave immediately and call 911 from outside.
- From a safe distance, check that the vent terminal and combustion-air inlet are not blocked.
- Do not run the appliance while a blockage is suspected.
Do not
- Do not put anything into the vent or air-intake openings.
- Do not remove or modify vent pipe.
How professional service helps
- A technician inspects the full vent and combustion-air path, clears blockages and checks the fan.
- 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.
Combustion fan (blower)
The combustion fan is the sealed boiler's lungs: it draws in combustion air, keeps the burner's gas-air mixture correct, and pushes flue gases out…
What a technician checks →Burner assembly
The burner assembly is where gas and air meet and burn inside a boiler, and its condition decides whether combustion is clean or makes carbon…
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 Viessmann Vitocrossal 300 (CU3A).
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
- Vitocrossal 300 CU3A Service Instructions, doc 5673 649 - 20, 03/2025 · 5673 649 - 20 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.