Error 4 flashes, 5 second gap, 5 long flashes

Safety precautions:

1. If the boiler is showing this over-temperature error, do not continue to use it. An over-temperature condition can cause scalding and damage internal components.

2. Where safe to do so, turn the boiler to the OFF position or to the reset position (use the control knob anticlockwise to RESET once) and isolate the electrical supply at the isolator. Only turn the gas supply off if you know where it is and can do so safely; otherwise leave gas isolation to the engineer.

3. Do not remove the boiler cover or attempt internal repairs. Internal components operate at mains voltage and involve gas; only a Gas Safe registered engineer should work inside the appliance.

4. If you smell gas, evacuate the property immediately, do not operate electrical switches, and call the emergency gas number before calling a heating engineer.

Initial checks a homeowner can do (non-intrusive):

1. After a safe reset, wait a few minutes and see if the fault returns. One single reset is acceptable; repeated resets are not recommended.

2. Check whether radiators and pipework in the heating circuit are hot or cold. If the pipework to/from the boiler is extremely hot but radiators are cold, suspect circulation failure (pump or closed valve).

3. Listen for the circulation pump when a demand is present (heating on). You should hear or feel the pump running briefly when the boiler fires. If the pump is silent or noisy, note that for the engineer.

4. Check system pressure on the boiler gauge. Very low pressure can stop circulation. Re-pressurise only if you know how and the boiler manual instructions — otherwise note the pressure reading for the engineer.

5. Bleed a radiator to remove trapped air which can cause poor circulation. If you are comfortable doing this, bleed the highest radiator on the system first and then check if circulation improves.

6. Check that remote room thermostats, TRVs and external controls are not stuck in a position that could confuse the system. Ensure any motorised valves are in the correct positions (if visible).

Specific diagnostic and fix steps (homeowner-level and information for the engineer):

Homeowner actions (safe, no internal access):

1. Carry out a single reset using the boiler control (turn control knob anticlockwise to reset) and observe: if the boiler runs normally and does not return to the error, monitor for several cycles — if it reappears, stop attempting resets and call an engineer.

2. If pump is suspected, try setting heating to call for heat and gently feel the pipes for temperature change. If the pump appears not to operate (no change and no noise), note this to the engineer.

3. If you have recently had power interruptions or works on the system, tell the engineer as that history can help diagnosis.

Information and checks a Gas Safe engineer will perform (do not attempt these yourself):

1. Measure actual flow and return temperatures with accurate instruments (thermocouples or clamp thermometers) while the boiler is firing to confirm whether measured temperatures match the flow sensor reading and to see if the 110°C threshold is actually reached.

2. Inspect and test the flow (supply) NTC sensor for correct resistance/temperature response and check wiring and connector integrity for shorts or intermittent connections. Replace sensor if faulty.

3. Check the safety temperature limiter(s) and max thermostat for activation or faults. Confirm whether any high limit cut-outs have tripped and reset/replace as required.

4. Verify pump operation and speed, check for a seized impeller, blocked system filter/strainer, closed zone valves or diverter valve faults that could prevent circulation. Also check pump wiring and speed settings.

5. Inspect the heat exchanger and condensate/pipework for blockages, scale or restrictions that could cause poor heat transfer and local overheating.

6. Check for improper installation or control faults (incorrectly configured hydraulic layout, bypass stuck closed, or incorrect pump wiring) that can cause the boiler to overheat.

7. Check the PCB and control module for faults and logged errors. If wiring or PCB faults are found, repair or replacement may be necessary.

When to call a professional and what to tell them:

1. Call a Gas Safe registered heating engineer if the fault persists after a single reset, if the flow temperature reaches very high levels, if you suspect the pump is not working, or if you notice leaks, smoke, smell of gas or other safety concerns.

2. When you call, give the engineer the exact model (Worcester Bosch CDi Highflow combi), the full error sequence (4 quick flashes, 5 second gap, 5 long flashes) and what checks you have already performed (pressure reading, radiator temperatures, whether you reset once, whether the pump was heard, etc.).

3. Do not attempt internal repairs, sensor replacements, PCB work, gas valve work or anything requiring access to the burner or gas connections unless you are a Gas Safe registered engineer.

Final note:

This fault can indicate either a real dangerous over-temperature condition or a sensor/wiring error. Because the boiler’s safety relies on correctly working sensors and circulation, a competent Gas Safe engineer should investigate and repair the cause rather than repeated DIY attempts. If the error returns after a reset or you are unsure at any stage, isolate the boiler electrically and call a professional.