Baxi [E.04.12] fault codewhat it means and how to fix it

E.04.12 is "false flame signal" in the 200/400 Combi manual and "ignition failure for monitoring parasitic flame" in the Combi 2 manual, which Baxi's professional page shortens to "ignition failure — flame monitoring". It is the newer displays' version of E384 on the three-digit Baxis: the flame-sensing circuit reports a flame at a moment when there cannot be one, so the board refuses to start an ignition sequence it cannot supervise. The 200/400 table is unusually full for this code. Causes: "short-term fluctuation of the mains", "ionization current present even though there is no flame", "the burner remains very hot (CO2 too high)", "defective gas/air unit".

Remedies: "press the RESET button for 5 seconds", "replace the ionization/ignition electrode", "set the CO2", "check the gas/air unit and replace if necessary". That five-second instruction is specific to this code on the 200/400 — every other permanent fault on the range resets in three, and five normally starts the de-aeration cycle — and it is quoted here because it is the kind of detail that gets generalised wrongly. The Combi 2 table is shorter: "check the ground circuit, check the power supply voltage".

Between them the picture is an electrode whose ceramic is fouled or cracked enough to leak sensing current to earth, a poor earth that leaves the sensing circuit floating, or a burner that has been running so rich and hot that the electrode still ionises after the valve closes. The last is a combustion setting problem and the reason the 200/400 remedy includes "set the CO2". There is no householder check beyond the single reset, and the reset needs the case left alone; if E.04.12 returns, the electrode and the gas setting are the engineer's job.