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Viessmann · Boiler

F.91 DIO module communication error

Medium severity Vitodens 200-W B2HE

What the control is telling you

Affected extension works in emergency mode.

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

Likely causes

  • Loose connection to DIO module or HMU

Test and repair steps

  1. Check connections to the DIO electronics module and to the HMU heat management unit.

Models that show this code

4 models in Vitodens 200-W B2HE

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

Other codes on the same units

  • F.765 Lead boiler communication error
  • F.77 BCU data memory error
  • F.8 DHW storage tank sensor short circuit
  • F.89 Internal circulation pump blocked
  • F.92 ADIO module communication error
  • F.94 PlusBus voltage error
  • I.56 External demand active
  • I.57 External blocking active

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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.