Error C4 273

Safety precautions:

- If you smell gas, leave the building immediately and call the gas emergency number. Do not operate electrical switches or the boiler.

- Do not open the boiler casing or attempt internal repairs unless you are a qualified Gas Safe engineer. Internal work involves live electrical circuits and gas components.

- Before touching any external isolators or electrical switches, dry your hands and stand on a dry surface. If you are unsure about isolating the boiler, wait for a qualified engineer.

Initial checks a homeowner can do (safe, external checks):

1. Note the exact fault code and any flashing light/triangle on the display. Write down the boiler model and serial number and the precise code (C4 273) for the engineer.

2. Try a controlled reset: press and hold the boiler reset or check/✔ button for about 3–8 seconds (follow your manual’s reset method). Release and allow the boiler to attempt restart. Observe whether the fan stops and the code clears.

3. Check the outside flue terminal and any air intake vents for obvious blockages (birds’ nests, leaves, snow, ice). Do not insert tools into the flue. If the flue was blocked and you safely remove the obstruction, reset the boiler and re-check.

4. Check the area around the boiler for signs of condensate freezing or blockages in external condensate runs in cold weather — a blocked condensate can cause other faults, so clear visible ice or debris from the external condensate trap/pipe if safe.

5. If the boiler remains noisy or the fan continues to run after reset, switch the boiler off at its fused spur or isolator and leave it off until a qualified engineer attends.

Specific diagnostic information and what a qualified engineer will do (for your information):

- The engineer will check logged fault history and run the boiler’s diagnostic/service tests (for example the fan test) to confirm continuous fan operation and to read any additional fault subcodes.

- They will inspect the fan, its wiring and connector for damage or loose connections, measure fan speed feedback (tach) and power to the fan, and test the fan motor electrical characteristics.

- They will inspect the control board/HCM and relays that drive the fan, re-seat or check the code plug/HCM connections, and may replace or repair the PCB/HCM if it is faulty. They will also check flue gas/safety thermostats and any interlocks that may affect fan control.

- If the fan motor is defective, the engineer may replace the fan assembly. If the control board or HCM is faulty, replacement or module swap may be required. They will retest the appliance after repair and reset any stored faults.

When to call a professional and what to tell them:

- Call a Gas Safe registered heating engineer if the error persists after a reset and external flue checks, if the fan runs continuously, or if you are uncomfortable performing the safe checks above.

- Provide the engineer with the boiler model, serial number, the exact fault code (C4 273), any recent service history, and what checks you have already performed (reset, flue cleared, etc.).

Additional homeowner guidance:

- If the boiler is running the fan continuously but otherwise appears safe and you have no heating or hot water, switch the boiler off at the isolator and wait for the engineer rather than leaving it running 24/7.

- Do not attempt to repair or replace internal components, wiring, or gas valves yourself. Only a Gas Safe engineer should perform internal diagnostics and repairs.

- After repair, ask the engineer to explain the root cause and any preventive actions (e.g., service interval, ensuring flue terminals remain clear).