Error S.3

Safety precautions:

1) If you smell gas (rotting/eggy smell), leave the property immediately, do not operate electrical switches, do not use telephones inside, and call the national gas emergency number from a safe location. 2) Switch the boiler off at the mains and the isolator if you need to inspect external parts. 3) Never remove the boiler casing or attempt internal repairs. Only a Gas Safe-registered engineer should work on gas, ignition or internal electrical components.

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

1) Check other gas appliances in the house (hob, gas fire) to confirm if gas is available. If none work, contact your gas supplier. 2) Check the boiler has power: is the fused spur on, is any RCD/fuse tripped? Check the boiler display for power. 3) Check the boiler pressure gauge: typical cold system pressure is around 1–1.5 bar. Very low pressure can stop operation — if you know how to repressurise your system safely and the manual permits, you may top up to the correct pressure. 4) Check room thermostat and programmer/timers are calling for heat and set correctly. 5) Check the condensate pipe (external frost/ice or blockage): in cold weather a frozen condensate pipe is a common cause of ignition lockout. If you see an external plastic pipe that looks frozen, thaw it by pouring warm (not boiling) water over the frozen section or use a hot water bottle until it flows. 6) Inspect the flue terminal outside for obvious blockages (nests, leaves, debris) but do not put your head over the flue. 7) Record the displayed error code(s) and any sequence (S.3 plus other codes) — this helps the engineer.

Safe diagnostic steps and simple fixes you may try:

1) Perform a controlled reset: after checking the simple items above, attempt one reset of the boiler using the reset procedure in your manual (typically press and hold the reset button for up to 10 seconds). Wait to see if the ignition sequence completes. Do not repeatedly reset the boiler more than two or three times — repeated resets can mask a real fault and may cause more wear. 2) If the condensate pipe was frozen and you have thawed it, try a reset once the condensate is flowing. 3) If system pressure was low and you have safely repressurised to the correct level, try a reset. 4) If an external factor (thermostat, timer, room control) was preventing a valid heat demand, correct that and reset.

When to call a professional (and what they will check):

1) If the error remains after the safe checks and a cautious reset, stop and call a Gas Safe-registered engineer. 2) Tell the engineer the boiler model, the S.3 code and any other fault codes or symptoms, and what checks you have already performed (power, gas to property, condensate thaw, pressure). 3) The engineer will carry out safe commissioning and diagnostic checks: verify gas supply and inlet pressure, test ignition transformer and spark, inspect and test electrode and ignition lead, test flame detection circuitry, check gas valve operation and resistance, inspect the PCB and wiring harness and any loose plugs, check safety devices (STB/flue temperature limiter), check flue and combustion, test flow/return NTC sensors if relevant, and inspect for blocked condensate or system debris. They will use appropriate meters and isolation procedures and only a qualified engineer should open and work inside the boiler.

Important reminders:

1) Do not attempt to adjust or replace ignition electrodes, gas valves, the PCB or internal wiring yourself. 2) If you suspect moisture on electronics or water ingress, isolate the boiler and call an engineer — do not run the boiler. 3) If the boiler keeps locking out after the engineer’s work, follow their advice; persistent ignition faults usually require part replacement or deeper inspection. 4) Always use a Gas Safe-registered engineer for repairs involving gas or internal boiler components.

If you need help finding a qualified engineer, contact Vaillant support or use the Gas Safe Register for a local, registered engineer. Provide them with the S.3 fault, any other codes, and details of your checks to speed diagnosis.