Viessmann Vitocal F.990 fault code

What it means

Short circuit, secondary-circuit temp sensor

The secondary-circuit temp sensor sensor reads open or short circuit.

Safety: Safe isolation before disconnecting sensors or working on the PCB.

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.990 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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