Viessmann [F.387] fault codewhat it means and how to fix it

Seven rows in the numeric table describe the flame-monitoring circuit finding an unwanted path to earth. F.387 — "Burner in a fault state — Earth fault, ionisation current. HBMU heat management unit faulty." — is the plainest. F.385 is "Short circuit, signal 1, ionisation current"; F.395, F.399 and F.401 read "IO electrode earth fault, HBMU heat management unit faulty"; F.403 and F.405 "Ionisation electrode earth fault, HBMU heat management unit faulty". Viessmann's naming puts the unit in the cause line, but the measures make the order of suspicion clear: "Check ionisation electrode and connecting cable" (F.387), "Check IO electrode for earth fault" (F.385, F.399, F.401, F.403, F.405), "Check ignition electrode for earth fault" (F.395) — and only then "If the fault persists, replace the HBMU heat management unit. Reset the appliance." The flame-monitoring input is a high-impedance measurement of a tiny current through the flame; any leakage to earth swamps it, and the unit cannot tell a leak from a fault in its own circuitry, so it names both.

The ordinary causes are physical: a cracked electrode ceramic letting the signal track to the burner door, condensate wetting the electrode after a blocked trap, a cable pinched under the burner cover with its insulation cut, or a plug part-seated. All are found by the contractor with the burner exposed and are dealt with by an electrode set and a cable check. The heat management unit is the answer only when a proven-good electrode on a dry, sound cable still returns the code. There is no householder action beyond the single reset the operating instructions permit for a locked-out burner and the call if it comes back; a flame-monitoring fault is not one to keep resetting, because the circuit that would detect a failed ignition is the one in question.