Error S.04

Safety precautions

1. If you smell gas: do not operate switches. Ventilate the area, turn off the gas at the meter or isolator if it is safe to do so, leave the building and call your gas emergency number and a Gas Safe engineer. Do not try to repair the gas system yourself.

2. Before touching the boiler: switch off the electrical supply at the isolator and allow the unit to cool. Never remove sealed covers or attempt to work on gas or ignition components unless you are a qualified engineer.

3. If the boiler is overflowing, leaking heavily, making loud banging noises, or showing safety cut-out fault codes (F.xx), isolate electrical and gas supplies and call a professional.

Initial checks a homeowner can do

1. Confirm there is a legitimate heat demand: check the room thermostat, programmer/timers and hot water settings are calling for heat and set above room temperature.

2. Observe the boiler display: S.04 means the burner is on. Note any additional F.xx fault codes or flashing symbols; those indicate faults that need attention.

3. Check radiators: are they warming evenly from bottom to top? If not, bleed radiators to remove trapped air.

4. Check boiler pressure on the gauge: typical operating range is about 1.0–2.0 bar (consult your manual). If pressure is very low, follow your manual to repressurise using the filling loop.

5. Inspect external flue and air intake: make sure the terminals are clear of debris, snow, bird nests or leaves.

6. Look for visible leaks, water around the boiler, or frozen/blocked condensate discharge pipe in cold weather.

Simple diagnostic and fix steps you can try (safe, non-intrusive)

1. Watch normal cycle: when S.04 appears, the burner should run until the set flow temperature is reached and then switch off. If it does that, no further action is needed.

2. If burner runs continuously and the system never reaches temperature: check thermostat and programmer settings, confirm TRVs are not closed, bleed radiators, and ensure the pump appears to be running (feel pipes for flow). If the pump is cold or not running, call an engineer.

3. If the boiler locks out with ignition or flame-related fault codes (for example F.28, F.29, F.67, F.27 or other F.xx codes) try one manual reset only (press fault-clear/reset for one second as per manual). If the fault returns or repeats after a single reset, do not keep resetting; note the exact fault code and call a Gas Safe engineer.

4. If you suspect the condensate pipe is blocked (common in cold weather) and you are competent to do so, check the external condensate discharge for ice or blockage and clear or thaw it carefully. If you are unsure, call an engineer.

5. If you notice a persistent flame where the burner should be off (or other unusual behaviour suggesting flame detection issues), switch the boiler off and call a qualified engineer—this can indicate a gas valve or PCB problem and is not safe to attempt yourself.

6. If pressure is low and you top up the system but pressure drops again, or you see error codes relating to sensors or circulation, stop and call an engineer.

When to call a professional

1. Call a Gas Safe registered engineer if you see any F.xx fault codes, repeated lockouts, smoke or strong gas smell, leaks, loud knocking or banging noises, no hot water or heat after basic checks, or if the pump, fan, gas valve, ignition or sensors may be at fault.

2. Provide the engineer with the boiler model, the S.04 status and any other displayed fault codes and symptoms you observed. This speeds diagnosis.

Final note

S.04 by itself normally means the boiler is simply firing to satisfy a heating demand and does not require repair. Use the checks above to confirm normal operation. For any safety-critical symptoms, persistent faults or anything involving gas, electrical or sealed internal components, always stop and contact a Gas Safe registered engineer.