Error E12

Safety first: If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency service. Before doing any inspection that involves touching the boiler, switch the boiler off at the mains and isolate the electrical supply. Never attempt to work on gas components, the gas valve or internal PCB—these must be handled by a Gas Safe engineer. Wear gloves and eye protection when working on the system and allow the boiler to cool if it has been running.

Initial homeowner checks you can do safely:

1. Note the exact error code and any other indications on the display. Take a photo and the boiler model/serial number for the engineer if needed.

2. Try a basic reset: switch the boiler off at the mains for 30 seconds, then switch back on and use the reset button (hold 3–5 seconds if required). If E12 returns immediately or after a short time, continue checks.

3. Check system pressure using the pressure gauge. If the pressure is below about 1.0 bar (typical target 1.0–1.5 bar cold), top up via the filling loop to bring pressure to the recommended level and retry. Low pressure can stop flow and trigger the switch.

4. Verify that relevant isolation valves are open: the boiler CH flow and return isolating valves and any external zone valve handles should be fully open. For combi boilers, ensure the domestic cold inlet isolating valve is open.

5. Call for heat or hot water while listening near the boiler: do you hear the pump running or the boiler attempting ignition? If you hear nothing, that indicates a pump or control power issue; if you hear the pump then there is circulation but the flow switch may still not be seeing correct differential flow.

6. Bleed radiators to remove trapped air. Air in the system can cause reduced flow or pre-circulation faults that leave the flow switch open.

7. Check visible pipework and the condensate/magnetic filter or any serviceable strainer for obvious blockages. If you have a magnetic filter on the return, it can become clogged — isolate system and clean if you are familiar with the process and have turned off power and water.

8. Visually inspect any accessible wiring and connector to the flow switch for loose connectors or corrosion (do not disconnect live wiring). If you need to touch wiring, isolate the mains first.

Specific diagnostic steps and safe attempts to clear the fault:

1. After checking pressure and valves, restart the boiler and call for heat while standing near the pump and flow-switch area. If the pump runs and pipes warm up quickly, but E12 persists, the flow switch may be faulty or set incorrectly.

2. If the pump does not run but you can hear the boiler trying to start or attempting ignition, suspect pump power/fused supply or wiring—do not open the pump electrical terminals yourself; note symptoms and call an engineer.

3. If you can access the flow switch body externally and it is mechanically accessible, sometimes a gentle tap (after isolating power) can free a stuck mechanism, or removing light debris from a strainer can restore flow. Only perform these actions if the components are external and designed to be serviced by a homeowner. Do not open or replace internal boiler components.

4. If steps above do not clear E12, record when the fault occurs (on hot water only, on heating demand, after long standby, etc.), take photos of the boiler display and wiring/labels, then contact a Gas Safe registered heating engineer. Provide them with the error code, observations about pump noise, system pressure, and any recent work that might have introduced air or dirt into the system.

When to call a professional:

Persistent E12 after carrying out the safe homeowner checks, any signs of pump failure (no sound, no pipe temperature change), visible leaks, electrical connector faults, or if you are unsure at any stage, call a Gas Safe registered engineer. A qualified engineer will safely test the flow switch, check pump operation and voltages, inspect internal wiring and the PCB, clear blockages in the primary loop, and replace the switch or pump if required. Do not attempt to replace the flow switch, pump, gas valve, or perform internal boiler wiring work yourself.

Extra notes: E13 is the complementary code for the flow switch stuck closed; mentioning which code your boiler shows helps the engineer. Keep a record of any interventions you performed and whether a reset temporarily cleared the fault; this information speeds up diagnosis for the engineer.