Overview
The 1 flash, 5 seconds off on a Worcester Bosch CDI Highflow combi means the boiler tried to ignite but did not detect ionisation (a flame signal) afterwards. Ionisation is how the boiler’s control electronics know the burner has lit — an electrode or flame sensor measures a tiny electrical current when gas is burning. If no ionisation signal is seen after the ignition sequence, the boiler will lock out to prevent unburnt gas being released. Common reasons include a faulty or dirty ignition electrode or its lead, a loose or damaged connector, a failed flame sensing circuit or PCB, a lack of gas to the appliance (meter/isolation valve issue), or air/combustion problems such as blocked flue or frozen/blocked condensate causing repeat lockouts. Severity is moderate to high: the boiler will not supply heat or hot water while locked out and there is a potential gas safety risk if the flame detection system is unreliable. Homeowners can perform basic, non-invasive checks and a single reset, but any internal work on ignition, gas or electrical components must be carried out by a Gas Safe registered engineer.
Possible Cause: No ionisation detected after ignition.
Troubleshooting Steps
Safety first:
1) If you smell gas, evacuate the property, do not operate electrical switches, and call the gas emergency number immediately. Do not try to repair gas equipment yourself.
2) Do not repeatedly reset the boiler more than once or twice — repeated lockouts indicate a real fault and repeated attempts can be unsafe.
3) Any work that requires removing the boiler cover, handling gas pipes, the gas valve, electrode, PCB or wiring must be done by a Gas Safe engineer.
Initial homeowner checks (safe and simple):
1) Note the fault pattern (1 flash 5 seconds off) and any cause code shown in the information menu. On many Worcester models you can press the spanner/return button to view a cause code — record that and the exact sequence for the engineer.
2) Try a single reset: either press the reset button for 3 seconds or turn the control knob to the reset position for 3 seconds as per your manual. Wait and see if the boiler fires and stays running. Do not keep resetting repeatedly.
3) Check the boiler and house power: make sure the external boiler power switch and any fused spur are ON and the consumer unit hasn’t tripped a fuse for the boiler.
4) Check gas supply: ensure the gas isolation valve to the boiler is open and test another gas appliance (hob) to confirm gas is available. If no gas to the property, contact your gas supplier.
5) Check condensate/pipework in freezing weather: if temperatures are below freezing, an external condensate pipe can ice up and block the system. If you suspect a frozen condensate pipe, carefully thaw it with warm (not boiling) water or a warm cloth. Clear any visible blockages at the flue terminal and make sure the flue/air intake is not obstructed by debris or nests.
6) Visual inspection only: look (without removing covers) for loose-looking external connections to the boiler, blocked flue terminal, or visible water pooling under the unit. Do not open the casing.
If the above checks do not clear the fault — call a Gas Safe registered engineer. Tell them: the exact fault sequence (1 flash 5s off), any cause code from the spanner menu, what preliminary checks you performed and whether you smelled gas.
What a qualified engineer will check and possibly do (for information only):
1) Check ionisation/flame sensing circuit and measure ionisation current and electrode continuity with suitable test equipment.
2) Inspect, clean or replace the ignition electrode and ignition lead; check the electrode mounting and gap and any ceramic damage.
3) Check all wiring/connectors for the electrode and PCB, and the condition of the gas valve and its control signals.
4) Verify gas pressure to the appliance and test for gas valve operation and possible leaks (EV1/EV2 leak tests if applicable).
5) Test the main control board (PCB/ionisation oscillator/heat control module) and replace if faulty.
6) Check for airflow issues, fan operation, blocked flue or condensate/drain issues that can cause repeated lockouts.
7) Run safety checks and reset the boiler correctly after repair; clear cause codes and confirm stable operation.
Final notes:
- Do not attempt internal ignition or gas-related repairs yourself.
- If the boiler repeatedly locks out with the same fault after an engineer visit, insist the ionisation/flame detection circuit and gas valve are fully tested and documented.
- If you are in any doubt about safety, switch the boiler off and seek a Gas Safe engineer immediately.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch CDI Highflow Combi Boiler.