HVACopediaViessmann · Boiler
F.350 Ionization current outside permitted range (BCU)
What the control is telling you
Burner in a fault state.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Likely causes
- Faulty BCU
Test and repair steps
- Replace the BCU burner control unit (using ViGuide, per the manual's replacement note), then reset the appliance.
Models that show this code
4 models in Vitodens 200-W B2HE
Other codes on the same units
- F.345 Temperature limiter tripped, auto restart after cooling
- F.346 Ionization current calibration error
- F.348 Gas modulation valve fault
- F.349 Air mass flow not detected correctly
- F.351 Ionization current outside permitted range (BCU)
- F.352 CO limit inside appliance exceeded
- F.353 Insufficient gas supply, output reduced
- F.354 Gas modulation valve tolerance outside range
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Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.62-73. 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.