Error 204 O

Safety precautions:

1. If you smell gas or suspect a gas leak: leave the property immediately and call the gas emergency number. Do not attempt any checks.

2. Turn off electrical supply to the boiler before touching or opening any covers or electrical connections. If you are unsure how to do this, stop and call a professional.

3. Do not attempt to repair gas, burner, flue, or sealed components yourself. Only carry out visual inspections and simple user-level operations.

4. Do not repeatedly reset the boiler more than 2–3 times. If resets don’t clear the code, call an engineer.

Initial homeowner checks (safe, simple):

1. Note the exact error code and any other symbols or codes shown. Record the boiler display and conditions when the fault happened (e.g., after thermostat change, after cutting power, during DHW use).

2. Try a single reset: use the boiler reset button and allow a few minutes to see if normal operation resumes.

3. Check the room thermostat and programmer: ensure there is an actual demand for heat and setpoints are correct. If thermostat setpoint is below current water temperature the boiler will wait.

4. Check the system pressure gauge: correct pressure is typically about 1–1.5 bar cold (check your manual). If pressure is very low, the system may behave unusually; top up via the filling loop only if you are comfortable doing so and know the correct procedure.

5. Check radiators and TRVs: ensure zone valves and radiator valves are not all closed and that radiators are not trapped with air. Bleed a radiator if it is cold at the top (do not over-bleed the system).

6. Listen and feel for the central heating pump operating: you should hear or feel a slight vibration on the pump or on the pipework (careful — pipes may be hot). If the pump is silent and radiators are cold while the boiler shows a high flow temperature, the pump may be failing or not powered.

7. Confirm isolation valves and motorised valves to the boiler are open (visual check only). If any valve has been shut after maintenance, reopen it.

Specific diagnostic checks and actions (if you are confident and safe to do visual checks):

1. After isolating electrical supply, visually inspect the wiring connector to the flow-temperature sensor (NTC) for obvious disconnection or damage. Do not disconnect or probe live terminals. Re-secure any loose connectors you can see without forcing or opening sealed parts.

2. If the flow temperature display is unusually high compared with expected water temperature, the sensor may be faulty. A sensor fault usually requires a qualified engineer to test and replace the NTC sensor or repair wiring.

3. If you suspect circulation problems: ensure pump speed selector (if accessible and labelled) is set correctly and not at the lowest setting; if the pump has a manual overrun or automatic function, ensure it is not locked. Do not dismantle the pump or remove covers — leave that to the engineer.

4. Check for airlocks by bleeding radiators and checking that the boiler flow/return pipes are hot (flow warmer than return). A very small temperature differential or very high flow temp with no flow suggests poor circulation.

5. If the boiler has external zone valves or an external controller, confirm those controls are calling correctly. A closed zone with the boiler flow still hot can create unusual readings.

When to call a professional:

1. If the 204 code remains after reset and the basic checks above, or if you find the pump not running, loose or damaged sensor wiring, repeated fault resets, or additional fault codes showing (especially safety limiter or flame/fan faults), stop and call a Gas Safe registered engineer.

2. Tell the engineer the exact error (204 O), what you observed (flow temperature reading, pump noise, pressure reading, whether reset was tried), and any recent work on the system. The engineer will have tools to test sensors, measure actual water temperature, check pump current and speed, inspect motorised valves, and replace sensors or faulty control components safely.

Important: do not attempt to access or repair burner, gas valve, flue, or internal control electronics yourself. For any persistent fault or where safety-related codes appear, arrange attendance by a Gas Safe registered heating engineer.