Error 2967 B

Safety first

1) If you are not confident working around boilers, pipes, or electricity, stop and call a Gas Safe registered engineer. Do not attempt gas or internal electrical repairs. 2) Isolate the boiler before opening any covers: switch off at the room thermostat, then turn off the boiler at the electrical isolator and allow it to cool. Do not touch live components or gas parts. 3) Never block or bypass safety devices, and do not try to run the boiler if you suspect a gas or electrical fault.

Initial homeowner checks you can do (no internal opening of the boiler)

1) Note when the fault appears: during CH (central heating) only, DHW only, or both. This helps narrow it down. 2) Check system water pressure on the boiler gauge and top up to the recommended level if it is low (usually around 1–1.5 bar cold for many systems). 3) Bleed radiators to remove trapped air and ensure circulation. 4) Check any external magnetic filter/strainer (magnaClean, etc.) and clear any debris there. 5) Check the system pump is running (you can usually hear or feel it) and if it is variable speed, try raising the pump speed one step to see if circulation/flow improves. 6) If you recently flushed the system or used chemicals (as in your notes), assume there may be dislodged sludge or air causing partial blockage—keep checking filters and the magnetic filter.

Diagnostic steps to try (safe, non-intrusive)

1) Observe behaviour and record displayed flow and return temperatures during a call for heat. If the flow temp rises very quickly while the other sensor reports much lower or much higher values, that reinforces a sensor or flow problem. 2) Try a single reset of the boiler (use the reset button) after you have done the basic checks. If the fault returns immediately or repeatedly, do not keep resetting. 3) Run the boiler in both CH and hot water modes (if applicable). If the fault appears in both, suspect a common item (flow sensor wiring, pump, or heat exchanger). If only on CH or DHW, that narrows the area to that circuit (e.g. diverter valve or cylinder circuit). 4) Open several TRVs or radiator lockshield valves to maximise flow and see if the trips stop. This helps determine whether poor circulation/flow is a cause.

More specific checks and likely causes

1) Sensors and wiring: loose connector, wet or corroded plug, or damaged cable can create wrong readings. A visible check of external wiring connections for the flow sensor (if accessible) is reasonable, but do not open the boiler unless isolated and you are competent. If connectors on accessible terminal blocks look corroded or moved, note that for the engineer. 2) Pump or impeller blockage: after flushing, debris can lodge in the pump impeller or heat exchanger passages. Signs include weak flow, rapid temperature rise on the boiler flow sensor, and cycling until the set flow is reached. 3) Heat exchanger restriction or scale: partial blockage restricts flow through the exchanger and can create a high temperature differential. 4) Safety/sensor trip due to rapid temperature rise: the boiler has gradient limitation to prevent the flow rising too quickly; if the control sees the flow temp rise much faster than the heat exchanger/safety sensor, it will pause the burner. This can be exacerbated by aggressive flushing or low boiler modulation settings.

Practical fixes to try before calling an engineer

1) Ensure magnetic filter/strainer is clean and pump speed set to a higher setting briefly to test improved circulation. 2) Fully open lockshield valves on a few radiators to force more flow and observe whether the fault stops. 3) If you can safely access and isolate the external wiring to the flow sensor and visually check the connector for corrosion or a loose plug, note or photograph it and gently reseat it if you are comfortable doing so (do not force anything). 4) Increase the boiler flow set temperature slightly as a temporary workaround to prevent rapid cycling (this masks the symptom but does not fix the cause). 5) Try running the boiler at a higher output briefly to see if stable running occurs; if it does, that again points toward flow restriction or the gradient limiter reacting at low modulation.

When to call a professional

1) If the fault persists after the simple checks above, call a Gas Safe registered engineer or Worcester Bosch service. They will have the tools to put the boiler into service/diagnostic mode, measure the actual resistance of thermistors, check sensor calibration, examine wiring at the control PCB, test pump flow with instruments, and inspect the heat exchanger and ignition/control electronics. 2) Tell the engineer you have seen 2967 B and describe recent system flush/chemical use, changes to the magnetic filter, pump speed changes, and the exact behaviour (cycling time and temperatures). That information is often very helpful and may point to debris in the system damaging the pump or blocking the exchanger.

Final note

Do not attempt gas valve, PCB, sensor replacement or internal boiler repairs unless you are a qualified engineer. Repeated resets or running the boiler with suspected flow restriction could worsen damage. For safety and correct diagnosis, a Gas Safe registered heating engineer or official Worcester Bosch service engineer should inspect internal sensors, wiring and the heat exchanger if the issue is not resolved by the basic homeowner checks.