Error S.21

Safety precautions:

- If you smell gas, get everyone out of the building and call the gas emergency number—do not attempt any checks. Ensure you have a working CO alarm installed and leave the boiler off if you suspect a flue or combustion problem.

- Before touching any internal parts: isolate the boiler from the mains electrical supply at the external isolator. Do not attempt any gas or high-voltage electrical repairs unless you are qualified.

- Do not run the boiler repeatedly if it is locked out; this can stress components and is unsafe.

Initial homeowner checks (safe, simple checks):

1) Note and photograph the exact display (S.21 and any accompanying F.xx codes). Those extra codes help identify the specific fault.

2) Try a normal reset: follow the boiler reset procedure in the manual (usually press and hold the reset/power button for up to 10 seconds). After reset, call for hot water and observe whether the fault returns.

3) Check that the boiler has mains power (external isolator/fuse) and that your household gas supply is on (check a gas hob or meter if safe to do so).

4) Check external flue terminal for obvious blockages (birds’ nests, leaves, snow/ice) and clear any visible debris safely from outside.

5) Check the condensate drain and discharge pipe (especially in cold weather) for frost or blockage; thaw a frozen condensate pipe safely if that is suspected.

6) Listen when you call for hot water: do you hear the fan spin up? If there is no sound or you hear a struggle/grinding, note this for the engineer.

7) Make sure vents and airways around the boiler are not blocked by clutter.

Diagnostic and repair steps (to be carried out by a qualified Gas Safe engineer):

1) Inspect the boiler log and full error history to confirm S.21 and any associated errors (for example F.32 fan fault, F.52 mass flow sensor, F.10/F.13 sensor shorts, or STB-related codes).

2) Isolate electrical and gas supplies, remove the front cover and carry out a visual check of the fan assembly, wiring harnesses, connectors and PCB plugs for corrosion, moisture, burn marks or disconnected plugs.

3) Check the fan connector is seated correctly on the PCB. Verify continuity of the fan wiring harness and check for broken wires.

4) Measure supply voltage to the fan during start-up. If voltage is present but the fan does not run, the fan motor is likely defective.

5) Test the fan hall sensor (or tacho) function and the fan motor resistance/insulation. Replace the fan assembly or hall sensor if faulty.

6) Inspect the flue gas temperature limiter (STB) and flue path for contamination/sooting or a stuck limiter. Clean or replace the STB if found faulty.

7) Verify any mass flow or air pressure sensors and their connections; replace or clean sensor/filter caps where applicable.

8) Check for moisture on the main PCB and signs of PCB faults. If electronics appear faulty, further testing or PCB replacement may be necessary.

9) Test associated temperature NTC sensors and their wiring if fault codes indicate sensor issues; correct any short circuits or swapped sensors.

10) After repairs, clear fault codes, run several start/stop cycles, confirm correct fan speed and stable combustion, and complete a safety and CO check before returning the boiler to service.

Important final notes:

- Because this fault affects combustion safety, we strongly recommend you contact a Gas Safe registered engineer if the simple homeowner checks do not fix it or if any internal inspection or replacement is required. Do not attempt to repair gas, electrical or sealed components yourself.

- When you call the engineer give them the exact error code(s), any observations from your listening/visual checks, and photos of the display and flue terminal to help speed diagnosis.

- If the boiler repeatedly locks out after reset do not keep resetting it; leave it turned off and book an engineer.