Error E12

Safety first:

1) If you smell gas, do not operate electrical switches or the boiler — leave the property immediately and call the gas emergency service. 2) If you are not confident working around boilers or plumbing, stop and call a Gas Safe engineer. 3) Always isolate electrical power to the boiler before opening covers; do not attempt internal electrical or gas repairs yourself.

Initial homeowner checks (safe, simple checks you can do):

1) Note the exact error code and any behaviors (lockout, lights, noises). Try a single reset: press and hold the reset button for 5–10 seconds and observe if the boiler attempts to restart. Do not keep repeatedly resetting if it returns immediately. 2) Check system pressure on the pressure gauge — aim for about 1.0–1.5 bar (manufacturer recommends 1–3 bar). If below ~0.8–1.0 bar, top up using the filling loop following your boiler manual, then reset. 3) Make sure central heating and hot water demands are present (room thermostat set above room temp, programmer calling for heat/hot water). 4) Check radiators for cold spots and bleed them to remove trapped air (airlocks can prevent flow). 5) Ensure any manual isolation valves on the boiler, pump or radiators are fully open.

Specific diagnostic steps you can try before calling an engineer:

1) Confirm circulation: with the heating demand on (thermostat high), listen for the pump running. You should hear/feel the pump hum. If the pump is silent but should be running, that indicates a pump or power/wiring issue — stop and call a professional. 2) After topping up pressure and bleeding radiators, try another reset. If the code clears and the system runs, monitor pressure and operation; if the fault returns, professional attention is required. 3) Check simple external obstructions: make sure pipework or the boiler condensate outlet and flue terminal are not blocked.

Basic checks on the flow switch (only if you are competent and have isolated power):

1) Isolate electrical supply to the boiler. Remove the outer cover following the manual. Identify the flow switch location (consult boiler schematic). 2) Visually inspect for broken wires, loose connectors, corrosion or a stuck paddle. 3) If you have a multimeter and are competent, you can check the flow switch contacts: with power still isolated, disconnect the switch connector and check continuity across the switch while someone else (or by changing controls) calls for heat so the pump runs — the switch should change state when flow is present. If you are not confident with this, do not attempt it.

4) If the paddle is stuck with debris or scale, very gentle cleaning may free it, but do not force parts or damage seals. Reassemble and test. If the switch still does not change state when there is clear flow, the switch is likely faulty and should be replaced by a Gas Safe engineer.

When to call a professional (must-do):

1) If the fault persists after basic checks (pressure correct, radiators bled, reset attempted). 2) If the pump is not running or you suspect pump failure, blocked heat exchanger, closed zone/diverter valves, or wiring/PCB issues. 3) For replacement of the flow switch, pump, or any internal components, and for any work on gas or mains electricity parts.

What to tell the engineer when you call: the exact error code (E12), what you have already tried (reset, pressure level, radiator bleeding), whether the pump is audible, and if the fault is intermittent or permanent. This helps them diagnose faster.

Final note: do not repeatedly reset the boiler if E12 returns — repeated lockouts can cause further faults. Any internal repairs, part replacements, or gas-related work must be carried out by a Gas Safe registered engineer for safety and warranty reasons.