Viessmann · Boiler

F.538 System return sensor open (SDIO)

Low severity Vitodens 200-W B2HE

What the control is telling you

No solar central heating backup.

Applies to: Vitodens 200-W B2HE 85, 120, 150, 199

Likely causes

Open circuit return sensor

Test and repair steps

Check sensors or connect the missing ones to the SDIO module.

Models that show this code

4 models in Vitodens 200-W B2HE

B2HE-120B2HE-150B2HE-199B2HE-85

Other codes on the same units

F.517 Remote control not working
F.527 Wrong HMU parameter set
F.528 Wrong BCU parameter set
F.530 Solar sensor missing or open
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.546 HC3 mixed supply 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.

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.