F26
Viessmann
Gas valve modulation / actuator fault
Gas valve modulation error, boiler may run only at low power or not at all
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Sticking gas valve actuator, Incorrect gas type configuration, Faulty gas valve feedback signal, Control board modulation circuit failure
How to Fix / Troubleshooting
Safety: Gas valve work must be done by licensed personnel only.
- Reset control: Power cycle the boiler and reset the fault once to rule out transient issues.
- Check gas type setting: Verify in the Viessmann service menu (installer level) that the correct gas type (natural gas or LPG) is selected. Only qualified personnel should change this.
- Observe operation: If the boiler lights but cannot modulate, or F26 recurs, stop using the boiler.
- Professional repair: A technician will test the gas valve actuator and feedback, and replace the gas valve or control board as required.
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Repair Difficulty
Professional Required
5/5
Required Part
Modulating Gas Valve
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