Error FLASH ON FLASH

Safety first

1) If you smell gas, or suspect a gas leak, get everyone out of the building and contact the gas emergency number immediately. Do not operate electrical switches.

2) If you are not a Gas Safe registered engineer do not attempt to work on gas or sealed components. Do not open the boiler and touch the PCB or internal wiring while the boiler is live. Isolate the electrical supply at the mains before any internal inspection.

Initial homeowner checks you can safely do

1) Check the boiler display and note the exact fault (flash pattern or any accompanying E-code). Take a photo for the engineer if you need to call one.

2) Try a controlled reset following the manufacturer instructions: use the reset button or move the selector to R and hold for five seconds (or follow the button procedure for your model). If the boiler was unlocked by a transient event this can sometimes restore normal operation.

3) Perform a full power cycle: switch the boiler off at its isolator and at the mains, wait 30–60 seconds, then turn back on and attempt reset.

4) Check basic supplies: ensure the mains fused spur or switch feeding the boiler is on and any RCD/fusebox hasn’t tripped. Check that the gas supply to the property is on (confirm other gas appliances if safe to do so).

5) Check visible condition: look for obvious water leaks or condensate pooling around the boiler, corrosion on external terminals or wiring, signs of burn or smell of electrical burning. Also check the condensate pipe (outside) hasn’t frozen or become blocked.

6) Check system pressure on the boiler gauge. If pressure is extremely low or high, that can cause other faults. Top-up only if you know how to use the filling loop safely and manufacturer guidance allows it.

If initial checks do not clear the fault

1) Do not try to access or repair the PCB yourself. Electrical and gas systems inside the boiler are live and dangerous.

2) Contact a Gas Safe registered heating engineer and provide: boiler model, exact fault description ("FLASH ON FLASH"), any other codes or lights shown, what you have already tried (reset, power cycle), and any observations about leaks, smell, or recent power interruptions.

What a qualified engineer will do (diagnostic and fix steps)

1) Visual and electrical checks: with power isolated, the engineer will inspect the PCB, connections and wiring for corrosion, loose connectors, burn marks, water ingress or blown board-mounted fuses. They will also inspect the casing and combustion chamber for signs that could have caused the fault.

2) Restore power and run live diagnostics: the engineer will observe the fault behaviour, check voltages at the PCB and onto actuators (fan, pump, gas valve) and measure sensor resistance/voltages (NTC thermistors, pressure sensors, flow switches) to confirm the PCB is receiving valid inputs.

3) Check peripheral faults that mimic PCB issues: verify fan operation, air pressure switch, flame detection and gas valve signals. A failing peripheral can trigger the PCB fault code; the engineer will isolate and test components to determine whether PCB or an external part is at fault.

4) Replace or repair: if the PCB itself is confirmed faulty the engineer will replace it with the correct manufacturer-approved PCB. They will transfer any user controls/knobs and ensure internal switches on the new board match the original (incorrect switch positions can prevent the boiler running). They will re-check connectors and ensure no wiring is trapped or damaged during refit.

5) Commissioning and safety checks: after replacement the engineer will run full safety checks: gas tightness test, combustion checks (if required), verify safe operation of ignition, flame detection, fan and pressure controls, and confirm no further fault codes. They will document work and advise on warranty/part details.

Important final notes

1) A single reset may temporarily restore operation, but repeat occurrences indicate a persistent fault and the PCB may need replacement. PCB faults can be caused by underlying issues (water ingress, failed sensors, power surges) so the root cause must be addressed to avoid recurrence.

2) PCB repairs and replacement must be done by a Gas Safe registered engineer. Working on gas appliances and internal electronics without the correct qualifications is dangerous and illegal in many jurisdictions.

3) Provide the engineer with any photos or notes of the fault pattern and recent events (power cut, lightning, freezing weather) to help faster diagnosis.

If the fault persists after a qualified engineer’s intervention, insist on a full diagnostic report and follow-up actions from the engineer or the manufacturer’s service support.