Error C4 273

Safety precautions:

- If you smell gas: leave the property immediately, do not operate electrical switches, ventilate if you can do so safely, call the gas emergency number and a Gas Safe engineer.

- If your CO (carbon monoxide) alarm sounds or you suspect a leak: evacuate and call the emergency services and a Gas Safe engineer.

- Do not open the boiler casings, do not attempt to repair or replace internal components (fan, gas valve, control board) yourself. Working on gas appliances is dangerous and must be done by a Gas Safe registered engineer.

Initial homeowner checks you can safely carry out:

1. Note the exact error code and any cause digits shown. Take a clear photo of the display for the engineer.

2. Try a controlled reset: use the boiler reset button (or turn the temperature dial to the reset position and hold for about 3 seconds per the manual). Wait and see if the code clears and if the boiler returns to normal. Do not keep repeatedly resetting more than 2–3 times.

3. Check your room thermostat, programmer and any smart controls to ensure they are not stuck calling for heat continuously. Turn the programmer/thermostat to off or to a lower setpoint and observe if the boiler stops calling.

4. Inspect the external flue outlet for visible blockages (leaves, bird nest, debris). Only clear external obstructions if you can do so safely from ground level; do not climb or force access if unsafe.

5. Listen for the fan: if the fan runs continuously even when there is no demand for heating or hot water, note that behaviour (make a short audio/video if helpful for the engineer).

6. Check system pressure on the boiler gauge and re-pressurise if it is significantly below the recommended level (consult your manual). Low pressure is not a primary cause of C4 273 but it's good practice to keep the system correct.

Specific diagnostic steps and actions:

1. After reset, monitor the boiler for 24–48 hours. If the error does not return and the system behaves normally, it may have been a one-off protective shutdown.

2. If the boiler continues to show C4 273 or the fan runs continuously, do not attempt to open the unit. Turn off the boiler at the isolator switch and at the time clock/programmer to stop continuous operation until an engineer attends.

3. If you switched the boiler off at the fused spur and the fan still appears to run, make a note of that — this is important information for the engineer.

4. Collect and record information for the engineer: boiler model (GB162), exact error code C4 273 and any three-digit cause code shown, whether heating and/or hot water is affected, whether the fan runs continuously, any unusual noises, and whether the flue looked blocked.

5. Contact a Gas Safe registered engineer or Worcester Bosch service. Tell them the boiler is showing C4 273 and give the details you recorded. Mention if the boiler is under warranty or service contract.

When to call a professional immediately:

- The code repeats after reset or the boiler will not restore heating/hot water.

- You suspect a blocked flue, persistent fan running, unusual noises, or other fault symptoms.

- You smell gas or your CO alarm activates.

Why a professional is needed:

- Persistent C4 273 commonly points to a fan motor fault, control board/relay fault, blocked flue or other internal issue. These involve live electrical and gas components and must be diagnosed and repaired by a Gas Safe registered engineer to ensure safe, correct repair. Provide the engineer with the error code, cause code, photos and details you collected so they can attend prepared.