- Boilers
- Gas boiler
65
Viessmann 65 on a Vitodens boiler: no flame signal at start
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On a Vitodens 100-W, code 65 means no flame signal, or too weak a signal, appears when the burner starts, and the boiler goes into a fault state. Check the gas valve is open, reset once and call a technician if it returns.
Safety first
Stop and check this first
- Code 65 concerns combustion or flue gas discharge: if a carbon monoxide (CO) alarm sounds or you feel headache, dizziness or nausea, get into fresh air, call emergency services and do not go back in until told it is safe.
- Do not keep resetting code 65: Viessmann warns that repeated resets in a short time with an unresolved fault can have fatal consequences.
- If you smell gas while code 65 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
Safe checks you can perform
- Look up code 65 on the display
- Check the gas valve or LPG supply
- Reset the burner once only
- Professional diagnosis of the failed flame start
Leave these tasks to a qualified professional
- Do not attempt this professional task: Inspect electrodes, ignition and condensate drain
- Do not attempt this professional task: Measure gas pressure and check the solenoid valve
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Inspect electrodes, ignition and condensate drain
What they doThe technician detaches the fan unit before reaching the burner and checks electrodes, leads, ignition and the condensate drain.
They must show youelectrode condition and spacing and the state of the condensate drain.
In writingwhich item was dirty, broken or out of position and the final start test.
Measure gas pressure and check the solenoid valve
What they doA qualified technician measures supply pressure, checks the gas flow switch and verifies the gas solenoid valve, as Viessmann requires for 65.
They must show youthe measured gas pressure and the solenoid valve response.
In writingthe readings and whether the gas supply explains code 65.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Look up code 65 on the display
- Who:
- you, using the panel buttons.
- How:
- open the fault list as the Vitodens user manual explains and note code 65 and whether the burner lockout symbol is flashing.
- What matters: knowing whether 65 appears alone or with other messages in the Vitodens 100-W list.
- If the code persists:
- if 65 is still listed, check the gas valve before attempting any reset.
- If it has cleared:
- if 65 is no longer listed, keep your note in case the Vitodens fails to light again.
- Evidence to keep:
- dated photo of the display showing code 65.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Check the gas valve or LPG supply
- Who:
- you, without opening the Vitodens casing.
- How:
- the Viessmann user manual says to open the gas valve when the burner does not light because fuel is missing; with LPG, check the remaining supply and reorder if needed.
- What matters: confirming the Vitodens has gas before suspecting the electrode or ignition behind 65.
- If the code persists:
- if the valve was open and gas is available, touch nothing else and report code 65 to the technician.
- If it has cleared:
- if the valve was closed and 65 clears once it is open, note this for the next service visit.
- Evidence to keep:
- gas valve position and LPG level checked against code 65.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Reset the burner once only
- Who:
- you, following the burner lockout section.
- How:
- note code 65, then hold the two buttons shown in the user manual for about 4 seconds; a progress bar confirms the reset has started.
- What matters: seeing whether the Vitodens starts normally or 65 returns on the next ignition attempt.
- If the code persists:
- if 65 returns, do not reset again: Viessmann warns that repeated resets in quick succession are dangerous.
- If it has cleared:
- if the home screen appears without 65, watch the next few starts and note any repeat.
- Evidence to keep:
- time of the reset and result of the start after code 65.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Professional diagnosis of the failed flame start
- Who:
- a qualified gas technician, working from the service manual for this Vitodens 100-W.
- How:
- measures gas pressure and checks the gas flow switch, solenoid valve, condensate drain, electrodes and ignition listed for 65.
- What matters: the technician identifying which of those checks explains 65 on your boiler.
- If the code persists:
- the technician should repeat start-up until the Vitodens lights without returning to 65.
- If it has cleared:
- ask for the inspected part to be recorded even if 65 does not recur during the visit.
- Evidence to keep:
- job sheet with the readings and the part inspected for code 65.
Removing the Vitodens burner, fan unit or electrodes is work for a qualified gas technician only.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Viessmann
- Product type
- Gas boiler
Known code variants
- Viessmann 65
- Viessmann error 65
- Vitodens 100-W 65
- Vitodens 65
- Vitodens fallo 65
What it means
Viessmann describes 65 as a missing or insufficient flame signal during burner start, with the boiler held in a fault state. The service instructions call for checking the gas supply (gas pressure and gas flow switch), the gas solenoid valve, the condensate drain, the ionisation electrode and its cable, the ignition and the ignition electrode condition. If the fault happens five times, the manual says the system locks out under a further message.
Warnings and stop conditions
- Criticaleditorial warning
Code 65 concerns combustion or flue gas discharge: if a carbon monoxide (CO) alarm sounds or you feel headache, dizziness or nausea, get into fresh air, call emergency services and do not go back in until told it is safe.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Highsource verified
Do not keep resetting code 65: Viessmann warns that repeated resets in a short time with an unresolved fault can have fatal consequences.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Criticalsource verified
If you smell gas while code 65 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026)
Symptoms and causes
Probable causes · in order
- Insufficient gas supplyPossible
Viessmann lists gas pressure and the gas flow switch as checks; the user manual adds a closed gas valve or an empty LPG supply as reasons for no ignition.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Condensate backing up in the boilerPossible
The checks for 65 include condensate backup and the condition of the condensate drain, which the technician inspects while preventing water damage.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Dirty, misplaced or broken electrodesPossible
The table covers the ionisation electrode and cable, ignition electrode distance and contamination, and a broken electrode ceramic.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Ignition system or gas solenoid valvePossible
The manufacturer also directs checks of the gas solenoid valve, the ignition and the leads between ignition module and electrode.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Virtual technician
Does 65 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
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