Vaillant aroTHERM F.821 fault code

What it means

Immersion heater flow sensor fault

The immersion-heater flow temperature sensor is not connected, or is faulty.

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

UK models this code applies to

From the Vaillant service literature we hold (Vaillant hydraulic station installation & maintenance instructions 0020257319 (VWL 57…127/5 IS), fault-message table) — not a scraped list.

Now the harder half: proving what caused it

F.821 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.

Free, no sign-up. Pick the model and follow Vaillant aroTHERM's own chart for it, one question at a time.