Error S.21

Safety precautions:

1) If you smell gas, turn off the gas supply at the meter, do not operate electrical switches, ventilate the area and call the emergency gas number and a Gas Safe engineer immediately. 2) Before touching anything on or inside the boiler, isolate the electrical supply at the fused spur or consumer unit. Do not open the boiler casing or try internal repairs unless you are a qualified engineer. 3) If you are unsure at any point, stop and call a Gas Safe registered engineer.

Initial checks a homeowner can do (safe, non-invasive):

1) Observe whether S.21 is only shown briefly during normal start-up. If it disappears and the boiler runs and produces heat/hot water, no further action is needed. 2) Note any other fault codes appearing (for example F.32 or other F‑codes) and the boiler behaviour (does it lock out, does the fan run continuously, is there no ignition?). 3) Check the external flue terminal and air intake for obvious blockages (bird nest, leaves, toys) from outside the building; clear any visible obstruction. 4) Make sure vents and air paths around the boiler are not blocked. 5) Verify the boiler has power (display lit) and that the gas supply to the property is on (check other gas appliances such as hob). 6) If your boiler manual gives a basic reset procedure, try a single reset per the manual and see if the boiler completes its start sequence. Do not perform repeated resets more than recommended.

If S.21 is persistent or the boiler does not start after initial checks:

1) Turn off the boiler at the electrical isolation point for 30 seconds and then turn it back on to see if the control sequence clears. 2) If the fan runs but the boiler will not ignite or it locks out, do not attempt to dismantle the fan or gas components yourself. A persistent fan-running status can mean the fan motor is running but the fan hall sensor or wiring to the PCB is faulty, the fan is blocked or seizing under load, or the main PCB is not receiving/confirming the correct feedback. 3) If you can safely access the outside flue, check there is no soot build-up or ice/condensate blockage in cold weather. Frozen condensate or blocked flue can disrupt operation. 4) Note any noise from the fan (grinding, rattling) which you can report to the engineer — noisy bearings point to replacement.

Information to collect for the engineer and expected professional diagnostics:

1) Which display codes are shown (S.21 plus any F codes), exact behaviour (fan runs continuously, boiler locks out, no flame), when the problem started and any recent changes (power cuts, cleaning, external works). 2) The engineer will isolate and remove the casing, check fan operation under load, check fan supply voltage and control signals, inspect the fan hall sensor and wiring plugs for poor connections, check the flue and pressure switches, and test the PCB for correct inputs/outputs. 3) Typical professional fixes include cleaning/clearing the flue or air intake, reconnecting or replacing a loose/damaged fan connector or wiring harness, replacing the fan assembly or hall sensor, or, if required, replacing a faulty PCB. Only a Gas Safe registered engineer should perform those checks and replacements.

Final note: S.21 on its own is usually a normal status during start-up. If the boiler runs correctly after the S.21 message there is no cause for alarm. If S.21 persists, the boiler fails to ignite, or other fault codes appear, stop further DIY attempts and contact a Gas Safe registered heating engineer to diagnose and repair the appliance safely.