HVACopediaViessmann · Boiler
A.20 Service interval could not be activated
What the control is telling you
Warning.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Likely causes
- Time/date not set
Test and repair steps
- Check time and date settings.
Models that show this code
4 models in Vitodens 200-W B2HE
Other codes on the same units
- A.12 Real-time clock battery flat
- A.18 Possible condensate back-up in heat cell
- A.19 Temperature limiter has responded
- Connection error Display link lost to HMU
- F.100 PlusBus voltage error
- F.104 External fault message input active
- F.11 Solar collector sensor open circuit
- F.12 Solar collector sensor short circuit
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Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.74. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.