BoilerFaultCodesViessmann 26 Fault Code
The Viessmann 26 fault code means: your boiler's computer has become stuck in a repeating internal test sequence and is failing to move into its normal heating mode.. It appears on 8 Viessmann models in our database.
Technical description: Boiler operates with continuous relay test
What causes the Viessmann 26 fault?
This fault occurs when the boiler's internal electronic controller gets trapped in a perpetual diagnostic loop during its start-up sequence. It is typically caused by a hardware failure on the main circuit board where the internal switches fail to release or communicate correctly, preventing the system from ever reaching the ignition stage.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the 26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 26 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
- Viessmann Vitodens 050-W manual (PDF)
- Viessmann Vitodens 111-W manual (PDF)
- Viessmann Vitodens 200-W manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026