HVACopediaViessmann · Boiler
F.546 HC3 mixed supply sensor open circuit
What the control is telling you
Mixer closes; pump keeps running.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Likely causes
- Open circuit sensor
- Wrong commissioning setting
Test and repair steps
- Check supply sensor of mixing valve 3.
- Measure about 3.3 V DC at the sensor input with sensor unplugged.
- Check commissioning assistant setting and ADIO rotary switch.
Models that show this code
4 models in Vitodens 200-W B2HE
Other codes on the same units
- F.538 System return sensor open (SDIO)
- F.540 Condensate back-up in heat cell
- F.544 HC2 mixed supply sensor open circuit
- F.545 HC2 mixed supply sensor short circuit
- F.547 HC3 mixed supply sensor short circuit
- F.548 HC4 mixed supply sensor fault (open circuit)
- F.549 HC4 mixed supply sensor short circuit
- F.57 Room temperature sensor open 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.62-73. Summarized in our own words; check the original manual for your model.
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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.