Error S.25

Safety precautions:

- If you smell gas, hear a hissing noise, or a carbon monoxide alarm sounds: turn off the gas supply at the meter (if safe to do so), switch off the appliance, leave the property immediately and call the emergency gas number. Do not use matches, phones, or electrical switches near the boiler.

- If you see leaks, flooding, excessive steam or flames, isolate electricity at the fuse box and call a qualified engineer.

- Never open the boiler casing or touch live electrical parts. Internal components and gas appliances must only be serviced by a Gas Safe registered engineer.

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

1. Note the display: is S.25 the only code or are there F-codes / lockouts shown? Make a note of any other codes shown and whether the boiler is supplying hot water or heating.

2. Check the boiler pressure gauge: normal cold system pressure is usually about 1.0–1.5 bar. If pressure is very low (<1.0 bar) top up the system using the filling loop following the boiler manual. Low pressure can cause circulation and pump-related faults.

3. Try a basic reset: switch the boiler off, wait 30 seconds, then switch on again or use the reset button per the manual. If the code disappears and stays away, monitor the boiler. Do not repeatedly reset a boiler that immediately returns to fault.

4. Check room/programmable thermostats, timers or external controls are calling for hot water correctly and are set as expected. If there is no DHW demand the boiler may be in a standby state where S-codes are shown.

5. Look and listen: any unusual grinding/noisy pump, gurgling in radiators, visible leaks, or signs of frost/blocked condensate outside the house? These are useful to report to an engineer.

If the status persists or hot water/heating is not working (advanced checks and information to give an engineer):

1. If S.25 is shown with other fault codes (F-codes), write them down exactly as shown. Many F-codes indicate specific sensor or safety cut-outs and are important for diagnosis.

2. Check whether the boiler attempts to ignite (you may hear the fan start, the gas valve operate, then ignition attempts). If the fan and pump run but the burner never lights or the boiler locks out, this suggests an ignition, gas supply, or control/sensor issue.

3. Check for low system water level causes: bleeding radiators recently, visible leaks, or a faulty automatic filling valve. Repeated low pressure can indicate a leak or expansion vessel problem.

4. If you have access to the external condensate pipe in freezing weather, ensure it is not frozen or blocked (a blocked condensate discharge can cause faults).

What a professional will check (do not attempt these unless you are qualified):

- Verify if S.25 is purely an informational state or related to a pump/fan overrun or fault, and read stored fault history in the boiler service menu.

- Measure flow and return temperatures and check flow/return temperature sensors and their wiring for faults or short circuits.

- Test the circulation pump for correct speed, flow and possible blockages or airlocks; remove and inspect if necessary.

- Check the diverter/3-port valve operation if the boiler is a system that switches between heating and DHW; a stuck valve can cause continuous pump operation.

- Inspect the water pressure sensor, pressure transducer and expansion vessel; confirm correct system pressure behaviour when the pump starts (to rule out F75-like symptoms).

- Inspect the fan and flue for blockages, and test fan motor and connections. Check for any ignition or flame detection faults if relevant.

- Check wiring, connectors and the PCB for communication faults or intermittent errors.

When to call a professional immediately:

- Any gas smell or carbon monoxide alarm activation.

- Repeated lockouts, persistent error codes that do not clear after a safe reset, loss of hot water/heating, visible leaks, loud unusual noises from the pump or fan.

Final note:

S.25 often appears as a status message meaning the fan and pump are running, and it may be harmless if transient. If S.25 is persistent, accompanied by F-codes, or the boiler is not providing hot water/heating, stop further DIY attempts and contact a Gas Safe registered engineer. Provide them with the exact codes shown, what you observed (noises, pressure, when it happens), and any recent work or changes to the heating system to speed up diagnosis.