Error 4 flashes, 5 second gap, 4 long flashes

Safety precautions:

- If you smell gas, immediately evacuate the property and call the gas emergency number. Do not operate electrical switches or the boiler.

- Before touching pipes or the boiler casing, switch the boiler off at the programmer and at the mains; keep in mind internal components can be hazardous and hot pipes can burn.

- Do not attempt to work on gas valves, the burner, the PCB or other sealed boiler internals unless you are a Gas Safe registered engineer.

Initial checks a homeowner can do:

1. Reset the boiler: follow the manual for your model (press and hold the reset button for 3 seconds or turn the control knob to the reset position for 3 seconds). Note what happens after reset and whether the fault returns.

2. Check the system pressure gauge on the boiler. Normal cold pressure is typically around 1–1.5 bar. If the pressure is very low, repressurise using the filling loop following the boiler manual. If you are unsure how, stop and call a professional.

3. Confirm whether the heating is on and if a heating demand exists (thermostat/programmer set to call for heat). If there is no demand, the boiler will not fire.

4. Check radiators around the house: are some fully hot or some cold? A very hot flow pipe and much cooler return on radiators is normal; the code implies return is abnormally hot compared to flow or expected values. Bleed radiators where the top is cold and the bottom hot to remove trapped air, then re-check boiler behaviour.

5. Listen and feel for pump operation (careful with hot pipes). You should hear a gentle hum when the pump runs and pipes should show circulation (flowing heat). If the pump appears dead, rattling, or stuck, note this for the engineer.

Specific diagnostic and fix steps for a homeowner (safe, non-invasive):

6. After reset, observe and record: is the boiler firing, does the fault come back immediately or after a short time, are there any other fault flashes or messages? This helps the engineer.

7. Check thermostatic radiator valves (TRVs) and any room thermostats: ensure they are open and not isolating most of the system. Closed valves or a mostly closed system can raise return temperatures.

8. If you have an accessible external zone valve or motorised valve, confirm it is moving when there is a heating demand (you may hear it or see an indicator). If a motorised valve is stuck on or stuck off it can affect flow/return temperatures.

9. If you have an infrared thermometer or thermometer probe, measure the boiler flow and return pipe temperatures (careful, pipes may be hot). A very small temperature difference (flow and return almost same high temperature) suggests poor heat extraction or little flow; a very hot return (near or above safety thresholds listed in manufacturer notes) confirms the problem is real rather than a bad sensor.

10. Bleed the system and re-check pressure and operation after bleeding. Airlocks and poor circulation will cause abnormal temperature behaviour.

When to call a professional and why:

11. If the fault persists after basic checks and resets, or if you suspect a faulty sensor, seized or faulty pump, blocked/scale-choked heat exchanger, stuck diverter valve or any internal boiler part issue, call a Gas Safe registered engineer. These components require specialist tools and correct safety testing.

12. Call a professional immediately if you see leaks, rapid pressure rise/drop, continuous boiler lockouts, or any additional fault codes or unusual noises.

What the engineer will likely do (for information):

- Verify flow and return temperatures with instruments, check sensor readings and wiring, test or replace the return NTC sensor if faulty.

- Test pump operation and bearings, check pump speed setting or replace pump if seized.

- Inspect motorised/diverter valves and system controls, look for closed circuits or stuck valves, check for correct zone operation.

- Inspect the plate heat exchanger for blockage or restriction and flush if required, check for scale/slag.

Record keeping:

- Make a note of when the error appeared, what you tried (resets, bleeding, repressurising), any other symptoms and other fault codes shown. This will speed diagnosis by the engineer.

Do not attempt internal repairs involving gas, burner, valve replacement, electrical connectors or PCB access unless you are Gas Safe qualified. If in doubt, switch the boiler off and contact a Gas Safe registered engineer.