Error S.4

Safety precautions:

1) If you smell gas or suspect a leak: get everyone out, do not operate electrical switches, call the gas emergency number immediately and call a Gas Safe engineer. 2) If the boiler is making unusual noises, producing soot/black residue, or showing flame or smell problems, turn the boiler off at the isolation switch and do not try to relight. 3) Never open the boiler casing, disconnect gas components, or attempt ignition/electrical repairs unless you are a Gas Safe registered engineer.

Initial homeowner checks (safe, non-invasive):

1) Note the exact code and when it appeared (during heating demand, after a restart, after power cut, after service, in cold weather etc.). 2) Try a controlled reset: follow the boiler manual reset procedure (typically press and hold the reset button for up to 10 seconds). Watch whether the boiler attempts to restart and whether S.4 returns immediately or after a short run. 3) Check the mains power and isolation switch feeding the boiler; ensure the boiler is getting electricity. 4) Check the domestic gas supply/mains to the property—are other gas appliances working? If no gas supply, contact your gas supplier. 5) Check boiler pressure on the gauge (typical resting pressure ~1–1.5 bar). Low pressure can cause faults—top up only if you know how and the appliance manual allows it. 6) Inspect the external flue terminal and immediate surroundings for visible blockages (birds' nests, leaves, snow/ice). Do not insert objects into the flue. 7) If condensate pipes are external and it’s freezing weather, check for frozen/blocked condensate pipe (thaw gently; do not force or dismantle). 8) Look for obvious water leaks, burnt wiring smell or signs of rodent damage to cable runs outside the boiler—do not open the boiler to inspect internals.

If S.4 persists after the initial checks and a reset:

1) Do not continue to try restarting the boiler repeatedly. Turn it off and isolate electrical power to prevent further attempts and potential damage. 2) Collect useful information for the engineer: exact fault code (S.4), display messages, when the fault occurs (on heating only or both heating and hot water), whether hot water is affected, whether code appeared after any work/service, last service date, whether you can see or smell combustion products, and any other related symptoms. 3) Contact a Gas Safe registered engineer and describe the symptoms and S.4. Explain the checks you have already done.

What the engineer will typically check (for your information):

1) Verify error history and live sensor readings on the PCB. 2) Test the flow temperature NTC thermistor and return sensor resistances (12kΩ at ~20°C is a common reference for some NTCs) and check connectors for corrosion/looseness or unplugged plugs. 3) Inspect the wiring harness and plugs between sensors/gas group and the PCB for short circuits, broken wires or bad connectors. 4) Check flame detection circuit: ignition electrode condition, ignition lead, spark generator and ionisation/flame sensing path and correct earthing. 5) Inspect the flue and measure flue temperatures; check and test the flue gas temperature limiter (STB) for soot, false trips, or failure. 6) Check for blocked condensate or flue, correct gas pressure and gas valve operation, and any faults in the gas valve or coding resistor. 7) Test the main PCB and replace if faulty; check for mains voltage issues that may affect PCB temperature. 8) Replace defective NTCs, wiring harness sections, STB, ignition components or the PCB as diagnosed.

Important notes and final recommendations:

1) S.4 is a safety-related shutdown. If a simple reset does not clear the fault permanently, treat the boiler as out of service until a Gas Safe engineer inspects it. 2) Do not attempt internal electrical or gas component repairs yourself. Only a registered gas engineer should carry out diagnostic tests, wiring repairs, sensor replacements, STB cleaning/replacement or PCB replacement. 3) When you call the engineer, give as much contextual information as possible (when fault started, weather, last service) to speed diagnosis. 4) If there is any sign of unsafe combustion (soot, smell, persistent flame instability) or repeated lockouts, isolate the boiler and call an engineer immediately.