Error S.23

Safety first

- If you smell gas (rotten egg/sulphur smell) or suspect a leak, do not operate electrical switches or the boiler. Turn off the gas at the meter if it is safe to do so, ventilate the property, leave the building, and call the emergency gas number for your country and a Gas Safe–registered engineer immediately.

- If you suspect carbon monoxide (headaches, nausea, soot, yellow flame), turn the boiler off, ventilate, evacuate and get help immediately. Fit a CO alarm if you don’t already have one.

- Do not remove the boiler casing or attempt internal repairs. Only carry out external, non-invasive checks described below.

Initial homeowner checks

1) Note the user display and exact behaviour: does the code reappear after a reset? Does the boiler try to ignite (you may hear the fan and a clicking spark) and then lock out? Record whether the fault happens on heating, hot water, or both.

2) Reset the boiler once following the manufacturer’s instructions (press and hold the reset button for a few seconds). If the fault clears and does not return, monitor the boiler — intermittent ignition faults can recur and still need a pro.

3) Check that the property has gas: confirm other gas appliances (hob, gas fire) work, check the gas meter supply hasn’t been turned off, and ensure any external gas isolator valve to the boiler is open. If other appliances do not work, contact your gas supplier.

4) Check electrical supply: is the boiler powered? Check the fused isolator or consumer unit hasn’t tripped and the boiler display is on.

5) Inspect the external flue and air intake: ensure there is no obvious blockage (birds’ nests, leaves, snow, debris) preventing combustion air or flue flow. Clear only external obstructions safely from ground level. Do not tamper with internal flue components.

6) Check the condensate discharge (if visible externally) for frost or blockage. A frozen condensate pipe can cause ignition/lockout faults in cold weather — if frozen, thaw gently with warm (not boiling) water and clear the outlet.

7) If the boiler or gas supply was recently turned off or worked on, air can be in the gas line. After confirming the gas supply is present elsewhere in the property, a Gas Safe engineer should purge and check gas pressures rather than homeowners attempting to bleed or adjust gas components.

Further diagnostics and what to avoid

- If you reset and can hear the ignition attempt, note whether there is a clicking noise (ignition spark) and whether the fan runs first. This information helps an engineer diagnose whether the problem is an electrode/ignition lead, ignition transformer, fan, or gas supply issue.

- Do not attempt to adjust or open the gas valve, change electrode gaps, or access the burner/PCB unless you are a qualified engineer. These are live, sealed or gas-containing parts and require specialist tools and certification.

- Do not repeatedly reset the boiler more than a couple of times; persistent lockouts indicate a fault that needs professional testing.

When to call a professional

- If the fault returns after a reset, if you cannot identify a safe external cause (gas supply, blocked flue, frozen condensate), or if you smell gas, call a Gas Safe–registered engineer or contact Vaillant support. The engineer will carry out safe checks: measure gas pressure, test and replace ignition electrodes/transformer or ignition leads, inspect the gas valve and PCB, check the ionisation/flame sensing circuit, test the fan and flue path and clear any condensate issues or combustion chamber water. Provide the engineer with the exact error code (S.23), when it happens, what you observed during the ignition attempt and any recent events (power cuts, gas work, freezing weather).

Bottom line: try a single reset and the safe external checks above. If the code persists or if you detect any smell of gas or signs of combustion problems, stop and call a Gas Safe engineer or Vaillant service—do not attempt internal repairs yourself.