Viessmann · Boiler

F.49 Flue gas temperature sensor open circuit

High severity Vitodens 200-W B2HE

What the control is telling you

Burner in a fault state; flue gas sensor disconnected or broken.

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

Likely causes

Open circuit on flue gas temperature sensor

Test and repair steps

Check the flue gas temperature sensor and its plug.
Reset the appliance.

Models that show this code

4 models in Vitodens 200-W B2HE

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

Other codes on the same units

F.471 Sensor circuit fault (call for heat)
F.473 HMU communication error
F.474 Program run monitoring error
F.477 Solar differential temperature monitoring
F.5 Sensor circuit fault at plug 33 (X6)
F.50 Flue gas temperature sensor short circuit
F.517 Remote control not working
F.527 Wrong HMU parameter set

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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.