Error D4 341

Safety first

- If you smell gas or suspect a gas leak, leave the building immediately and call the national gas emergency number. Do not attempt repairs.

- Only a Gas Safe registered engineer should work on gas parts, the gas valve, burner or internal wiring of the boiler. If you are not competent with electrical work, do not open the boiler casing.

- Before doing any visual checks, switch the boiler off at the isolator and allow it to cool. Switch off mains power if you will be handling wiring or connectors.

Initial homeowner checks (safe, non-invasive)

1. Note the fault: press reset once and record if the fault returns, and how long after calling for heat it appears.

2. Check the system water pressure on the boiler gauge. Aim for roughly 1.0–1.5 bar when cold. If pressure is low, re-pressurise following the user manual and see if behaviour changes.

3. Check thermostats, programmer and room stat settings. Make sure there isn’t a rapid on/off demand causing odd cycling. Disable eco or energy-saver modes temporarily to test.

4. Check radiators and system circulation: bleed any radiators with cold spots, ensure TRVs and radiator valves are open and zone valves (if fitted) are calling. Run the boiler and check if all radiators gradually warm; poor circulation can cause temperature sensing anomalies.

5. Run the boiler’s automatic air-purge or commissioning mode if your model has it and you’re comfortable following the manual. Air in the primary circuit can cause unstable temperature readings.

More technical diagnostic steps (only if competent and comfortable with basic electrical checks)

6. Observe the flow temperature reading (quick info or diagnostics menu) and log how quickly it rises after burner ignition and how far above the set point it goes. A steady, modest overshoot is different from a very rapid spike.

7. Check pump setting: If the pump is set to the lowest head, circulation may be marginal. If you can safely adjust the pump head, try increasing it one step and observe whether flow temperature stabilises. Do this only with power off to the pump and following the pump/boiler manual.

8. Inspect external visible wiring and connectors to the boiler (not internal wiring) for obvious damage. If you can safely access the small plug connection for the flow sensor and you are competent with basic isolation, switch off mains, remove the boiler casing and check that the sensor plug is fully seated and undamaged. Refit and re-test. If you are unsure, stop and call an engineer.

9. Consider blocked heat exchanger or scale: restricted flow through the primary heat exchanger can cause unexpected temperature behaviour. Signs include poor radiator heating, frequent thermostat overshoots or noisy flow. These conditions usually require an engineer to flush or descale the system.

10. If the fault only appears intermittently or after resets and you have tried the above, it increasingly suggests a failing flow sensor, code plug/KIM or a control module fault. These components require a Gas Safe engineer to test and replace.

When to call a professional

- Call a Gas Safe registered engineer if the error persists after basic checks (reset, pressure, bleed, pump adjustment) or if you are not comfortable opening the boiler. Professional checks will include verifying sensor resistance/voltage, checking the plug/KIM, testing the pump and circulation under load, and inspecting the control board.

- Do not attempt to replace gas or combustion components yourself. Repeated lockouts indicate the underlying fault should be fixed to avoid further damage to the boiler. Make a note of when the fault occurs and any conditions that coincide with it (e.g. cold start, after bleeding, when certain zones call) to help the engineer diagnose more quickly.