Viessmann · Selected Vitodens and Vitotronic ranges
D4Safety equipment or internal BUS fault; meaning depends on model
This code applies to Vitodens 100-W, Vitodens 200-W, Vitodens 222-F, Vitotronic 100, Vitotronic 200, Vitotronic 200-H, Vitotronic 300, Vitotronic 300-K, Vitocontrol-S. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- Do not remove the boiler case.
- Record the full code and model, then arrange a Gas Safe registered engineer.
How an engineer may fix it
Follow the model-specific fault-finding procedure, test the indicated circuit or component, repair the cause and recommission the appliance.
Viessmann warns that the same code can have different meanings on different models. The exact manual must take precedence.
Manufacturer source: Viessmann UK — Fault Code Checker. Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.