Error FLASH ON ON

Safety first: Do not work on gas or live electrical parts unless you are qualified. Turn the boiler selector to OFF and switch off the boiler at the external isolator or at the household fuse box before inspecting. If you smell gas, evacuate the property and call the gas emergency service immediately. Never attempt to modify or replace gas valve components yourself — a Gas-Safe engineer must do any gas-side work.

Initial homeowner checks (safe, non-invasive):

1) Note the exact fault display (FLASH ON ON) and the conditions when it appears (after reset, on power-up, intermittently). Take a clear photo of the display and any status LEDs — useful for the engineer. 2) Try a single controlled reset using the boiler reset button or the selector as per the manual. Wait a few minutes to see if the boiler attempts to run and whether the error returns. Do not repeatedly reset more than 1–2 times. 3) Check the boiler’s electrical supply and RCD/trip fuse in your fuse box; ensure the fuse supplying the boiler has not blown and the switch is ON. 4) Check for obvious signs of water leaks around the boiler or condensation inside the casing. Water on or under the PCB is a common cause. 5) Make sure external controls/thermostat batteries are OK and any timers are calling the boiler.

What you can inspect if you are comfortable and the boiler is isolated (but avoid internal repairs):

- Visually inspect the boiler exterior and around the condensate pipe for freezing or blockage. - With the boiler powered OFF and isolated, you may remove the outer cover to inspect for obvious damage only (burnt components, corrosion on the PCB, loose connectors). Before touching anything, photograph the wiring connections so they can be returned exactly as found. Do not probe or touch live connectors.

Diagnostic steps a qualified engineer will perform (do not attempt unless certified):

- Verify the fault code and reproduce the fault sequence safely. - Restore and isolate power, then check incoming mains, internal fuses, and low-voltage supplies to the PCB with a meter. - Inspect PCB for visible damage: burnt tracks, blown components, dry or cracked solder joints, corrosion from water ingress. - Check connectors and harnesses for secure connections and damaged pins; check grounding. - Verify related components (sensors, flow switch, fan, pump, gas valve) for faults that can present as a PCB error; sometimes a downstream failure can appear as a PCB problem. - Test communications and control outputs (ignition, gas valve, fans) and measure voltages and signals to confirm whether the PCB or a connected component is at fault. - If corrosion or water damage is present, the engineer will investigate the leak source (pump seal, heat exchanger, pipework) and recommend repair before fitting a new PCB. - If the PCB is confirmed faulty, the engineer will replace it with the correct OEM or approved replacement, set any jumpers/DIP switches to the correct positions, transfer/reset the front-panel reset button if applicable, reattach wiring per the photos and service manual, then run full operation and safety checks.

When to call a professional and next steps: If the initial safe checks (power, reset, visible leaks) do not clear the fault, arrange a visit from a Gas-Safe registered heating engineer or Baxi service agent. Do not attempt to operate the boiler or repair gas/electrical components yourself. If the boiler is under warranty, contact Baxi or your installer before any component replacement. Provide the engineer with the exact error (FLASH ON ON), photos of the display and any PCB damage you observed, and any sequence notes (when it began, recent power cuts, evidence of leaks).