Error C4 273

Safety precautions:

- If you smell gas, leave the property immediately and call the gas emergency number. Do not attempt any checks on the boiler.

- Turn off electrical power to the boiler at the isolator switch before doing any external inspections. Do not remove the boiler casing or work on internal components unless you are a qualified engineer.

- Work from a stable position when checking external flues and keep children and pets away while you check things.

Initial checks a homeowner can do:

1) Note the exact error code, any accompanying lights or display messages and whether the code is persistent or intermittent. This information helps an engineer.

2) Try a boiler reset following the manufacturer’s procedure (usually a reset button or by switching off, waiting 30–60 seconds and switching back on). If the C4 273 clears and does not return, monitor for recurrence.

3) Inspect the external flue terminal for obvious blockages (leaves, nests, debris, bird caging or snow/ice). Clear only external obstructions by hand or with a soft brush; do not insert tools into the flue.

4) Check for recent events that could have affected the boiler (power cuts, storms, building work) and note whether any unusual noises were heard (continuous fan noise when boiler is off).

Specific diagnostic and fix steps (homeowner actions and items to pass to an engineer):

A) Homeowner-level steps:

- If the flue was blocked and you cleared it, reset the boiler. If the code does not return, the issue may have been a temporary external obstruction.

- If the condensate pipe is routed externally and frozen (common in cold weather), thaw it with warm water or a warm cloth and then reset. Frozen condensate usually causes other codes but is worth checking if pipes are external and you see ice.

- Record when the error occurs (continuous, after a power cut, only at night, after hot water use etc.). This pattern helps diagnostics.

B) Steps that require a Gas Safe registered engineer:

- The engineer will check fan operation and control signals: measure fan supply voltage and speed, inspect fan motor, bearings and wiring harness, and run component tests from the boiler service mode.

- Inspect and test the airflow monitoring circuit and related sensors, and check connectors between the fan, HCM (heat control module) and main PCB for corrosion, loose pins or damaged cables.

- If the fan motor is seized, running on, or running when it shouldn’t, the fan assembly is normally replaced rather than repaired. If the control electronics or HCM are faulty, these modules may need replacing.

- The engineer will also check for related faults that can cause abnormal fan behaviour (faulty control board, incorrect coding module, or failed relay outputs) and will clear fault history and verify correct operation over a test cycle.

When to call a professional:

- If the C4 273 returns after a reset, or if you cannot safely access/clear the flue terminal, call a Gas Safe registered engineer.

- Always call a Gas Safe engineer if you suspect internal faults, need internal access, or if the boiler will not start and shows the same error. Do not attempt to remove the boiler casing, test internal voltages, or replace electrical components yourself.

Final notes:

- Keep a log of any resets, flue checks and the timing of fault occurrences to give to the engineer.

- This fault typically requires trained inspection and electrical/mechanical testing, so while basic external checks are appropriate for a homeowner, plan to book a Gas Safe engineer if the problem persists.