Viessmann Vitocal F.793 fault code

What it means

Phase L3 missing on instantaneous heating water heater

Safety: Safe isolation, then TREAT THE DC LINK AS LIVE. The electrolytic capacitors hold a lethal charge after the supply is off. Discharge through a resistor across the + and − terminals (Gree specifies approx. 100 Ω 20 W, Toshiba approx. 100 Ω 40 W) and confirm with a meter on DC before touching the board. Never short the terminals with a screwdriver.

UK models this code applies to

From the Viessmann service literature we hold (Viessmann One Base service instructions 6194799 (2022), fault & warning tables (Vitocal 150-A/151-A)) — not a scraped list.

Now the harder half: proving what caused it

F.793 names a symptom, not a root cause. In the app, the same code opens a guided investigation: what to measure first, the expected values for your exact model, and an honest verdict — including “nothing in this machine is broken” when that's what the readings say.

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