Error D.1

Safety first

- If the boiler displays a permanent fault or locks out repeatedly, switch it off and isolate the gas at the appliance isolator only if you are uncomfortable; otherwise leave it on and phone a Gas Safe engineer. Do not attempt to work on gas components or exposed mains wiring. Turn off mains power before inspecting or touching any internal controls if you are competent and it is safe to do so.

- Never block or remove the flue. Ensure good ventilation in the cupboard/room.

Initial checks a homeowner can safely do

1) Note and photograph the display message and any sequence of codes or illuminated LEDs. Record the conditions when it fails (time since boiler turned off, room stat set point, whether DHW was used, radiator/valve positions).

2) Check system water pressure on the boiler gauge. Pressure should normally be around 1–1.5 bar when cold. If pressure is very low (<0.8 bar) or very high, top up or isolate and call engineer.

3) Verify room thermostat and programmer/timer settings and batteries. Make sure the thermostat isn’t sending rapid on/off demand. Temporarily raise the thermostat a few degrees to see if behaviour changes.

4) Bleed radiators to remove trapped air and ensure good flow; check for cold spots indicating circulation problems.

5) Listen to the pump: it is normal for the pump to run for a few minutes after burner stops. Note if the pump continues running for excessive periods (many minutes to hours) or the pump runs even when the boiler is supposed to be off at the mains.

6) Check the boiler external wiring visually for loose leads or signs of recent incorrect work (especially if a new pump was fitted). Do not touch wiring while powered.

Specific diagnostic steps and what they indicate (safest sequence)

1) Reset the boiler fault once and observe: press the fault/reset button briefly and watch behaviour. If the boiler restarts normally from cold but misbehaves only when warm, this suggests a temperature sensing or control logic issue rather than a completely failed component.

2) Reproduce the problem: with the boiler warm (within 30 minutes of shutdown), call for heat and watch the sequence. Compare to the cold-start sequence. Note whether the burner stops prematurely, whether the pump runs continuously, and whether the boiler goes to a pump-overrun lockout or safety shutdown.

3) Check the 3‑port/motorised valves visually: with heating demand, the three-port valve head should move. If you have a Y- or S-plan system, observe if the valve is fully switching and not stuck halfway (a stuck valve can cause the boiler to behave oddly). Do not disassemble valve; only observe movement.

4) Expansion vessel check: test the Schrader (air) valve on the expansion vessel with a finger or tyre gauge if present. If you cannot get any air out, the vessel may be waterlogged or the valve may be seized. However, do not attempt to recharge the vessel unless you have the right tools and experience. Note that a waterlogged vessel can cause pressure/short-cycling issues.

5) If a pump was recently replaced, ask the installer whether the pump wiring was replaced exactly as original. Incorrect pump wiring or a pump with a built-in run-on wired wrong can cause the pump to run when it shouldn’t. If you are not competent with mains wiring, have the installer re-check wiring.

6) Fault isolation for controls: if comfortable and safe, you can isolate the room thermostat or programmer (by setting the boiler to be permanently in demand mode according to the boiler manual) to test whether the external control is causing rapid on/off commands. Only do this if you understand how to revert the change.

When to stop and call a professional

- Call a Gas Safe registered engineer if: the boiler locks out into safety mode repeatedly; the pump runs for excessive periods; the boiler overheats or reaches abnormal temperatures; you find signs of coolant leak or cannot restore normal pressure; you suspect faulty wiring, PCB, or temperature sensors; or you are uncomfortable performing any of the steps above.

What a competent engineer will check/likely do

- Read live diagnostic/status logs from the boiler to see pump/fan/run sequences and exact codes. Test NTC temperature sensors (flow and return), the internal bypass, and the heat-exchanger temperature behaviour.

- Verify correct pump wiring and operation, test the pump run-on circuitry and any linked boiler PCB outputs. Replace or rewire if incorrectly installed.

- Inspect and, if necessary, replace the 3‑port valve or its microswitches if stuck or corroded. Clean or replace the valve motor head.

- Check PCB and control logic for faults, and test/replace thermostat/timer and any room controls if needed.

- Test and recharge/replace the expansion vessel if it is waterlogged or has lost charge.

- Pressure-test and purge the system, and verify correct bypass settings and flow through the heat exchanger.

Final notes

- Short normal pump overrun is protective and not an immediate safety concern. Repeated overrun, unmanaged temperature rises, repeated lockouts or inability to restart reliably indicate a fault needing a Gas Safe engineer. Provide the engineer with photos, timing notes and any display codes you recorded to speed diagnosis.