Error 5 flashes 5 seconds off

Safety first

1) If you smell gas: turn off the gas supply at the meter if safe to do so, ventilate the building, do not operate electrical switches or the boiler, leave the property if necessary and call your local gas emergency number immediately. Do not attempt to repair any gas components yourself.

2) If you do not smell gas: isolate the boiler electrically (switch off at the fused spur or mains) before doing any internal inspection. If you are not competent or comfortable, do not open the cover — call a Gas Safe engineer.

Initial homeowner checks you can safely perform

1) Try a single reset: use the boiler’s reset function once and observe. Note whether the error returns, and when (immediately, after running hot water, after heating call). Do not repeatedly reset the boiler.

2) Check gas supply: confirm other gas appliances (hob, oven) are working and the gas meter is on. If there’s no gas supply to the property, report to your gas supplier.

3) Check the boiler room and flue termination: look for obvious blockages, nests or water ingress at the flue outlet and ensure the area around the boiler is dry and ventilated.

4) Visual inspection from outside (no internal access): check for obvious external signs of damage, corrosion or loose wiring to the boiler. Check condensate pipe for blockages or icing if it’s winter.

5) Record details: write down the exact flash sequence, when it happens (after shutdown, before start, during operation), any audible gas smell, and any recent service or work on the boiler — this information helps the engineer.

Specific diagnostic and fix steps (what an engineer will do — do not attempt unless Gas Safe qualified)

1) Verify the fault: engineer will hook up diagnostic tools and confirm the 5-flash condition and any stored fault codes and timestamps in the control PCB.

2) Check ionisation/flame sensing circuit: inspect and test the electrode (flame sensor) and ignition electrode for correct position, gap, carbon/soot, moisture, and secure connections. Clean or replace the electrode and leads if faulty or contaminated.

3) Inspect wiring and earth: test the ionisation lead, connectors and earth/ground continuity. A shorted ionisation sensor or poor earthing can give false ionisation signals.

4) Check combustion chamber and flue: inspect for water or soot ingress that could cause conduction or false signals, and check the fan and flue system for correct operation.

5) Gas valve leak tests (EV1/EV2): the engineer will perform the built-in leak tests on EV1 and EV2. A failed leak test indicates the valve is not sealing and can allow gas leakage into the burner or manifold — a potential safety risk. Gas valve repairs or replacement must only be done by a Gas Safe engineer.

6) Control electronics: if electrodes, wiring and gas valve test within limits, the PCB/burner control unit or HCM/KIM module could be at fault. The engineer will check the control board and replace if necessary.

7) Final functional tests: after any repair or replacement, the engineer will repeat ionisation and leak tests, run ignition cycles, verify safe operation and clear codes.

When to call a professional

1) Call a Gas Safe registered engineer as soon as possible if the error persists after a single reset, if the error appears repeatedly, or if the boiler is locking out. Do not run the boiler unattended.

2) Call immediately and treat as an emergency if you smell gas or suspect a leak.

What to tell the engineer when you call

1) Model and serial number (Worcester Bosch CDI Highflow Combi), the exact light sequence (5 flashes, 5 seconds off), whether it happens after shutdown or before start, any smell of gas, and any recent servicing or changes.

Final note

This error can indicate a potentially dangerous condition (gas valve leak or false flame detection). Aside from the basic, non-intrusive checks listed above, do not attempt to repair or replace gas valves, the ignition electrode assembly, or control electronics yourself. A Gas Safe registered engineer must carry out capacitive/ionisation current measurements, gas-valve leak tests and any component replacements.