Error C6

Safety first

- If the boiler has locked out display C6, you can attempt safe, non-invasive checks but do not carry out gas or live-electrical repairs unless you are a qualified engineer.

- Turn off the boiler at the control and switch off the electrical supply at the fused spur before opening the casing. If you smell gas or suspect a leak, stop immediately, ventilate the area, do not operate electrical switches, and call the gas emergency service.

- Use eye protection and a non-metalic tool if probing near plastic parts. Never bypass safety devices.

Initial homeowner checks (non-invasive)

1. Try a reset: follow the boiler manual/reset button. If the fault is transient the code may clear. Do not repeatedly reset—if it returns, further action is needed.

2. Check the external flue terminal: from outside, ensure the flue outlet is free from debris, birds’ nests, snow/ice or anything obstructing flow.

3. Check for recent freezing/condensate problems: look for blocked or frozen condensate pipework and clear if safe to do so.

4. Check for water or moisture inside the boiler casing or visible corrosion around the fan connector area — moisture can cause low-speed or incorrect readings.

5. Check the boiler has been serviced in the last 12 months; lack of service can allow build-up of dust/debris affecting the fan.

If you are comfortable with simple DIY and have basic tools (but not trained to work on live gas/electrics)

1. Isolate electrical supply at the fused spur and confirm the boiler is de-energised. Remove the outer casing per the manual.

2. Visually inspect the fan and impeller: look for obvious obstruction, heavy dust, debris or damage to the blades. Try to spin the impeller gently by hand — it should turn freely with minimal resistance (do not force it).

3. Inspect the fan wiring harness and connector: look for loose pins, burnt contacts, corrosion, or a pulled-through wire. Re-seat the connector if it is loose.

4. If the connector, lead or wiring loom looks damaged, it may be possible to replace the fan lead/connector part. Only fit identical replacement parts and follow the manufacturer’s guidance. If unsure, stop and call a Gas Safe engineer.

5. Replace casing, restore power and attempt a controlled restart. Observe whether the fan starts and whether the C6 returns.

Diagnostic steps that should be carried out by a qualified engineer

1. Live electrical tests and voltage checks at the fan terminal and continuity checks of the fan cable with a multimeter. These involve live working and must be done by a Gas Safe / electrically competent technician.

2. Remove and bench-test the fan motor assembly for correct operation, bearings wear, shaft play and current draw. Replace the fan assembly if bearings are worn or motor performance is outside specification.

3. Check and test the air pressure switch and its tubing for blockages or leaks that could give a false airflow/fan-speed reading.

4. Inspect and test the PCB and fan-control electronics; if the PCB is faulty it may misread the fan speed. Record service settings before replacing any PCB and restore them after replacement.

5. Perform a flue flow test and combustion safety checks after any fan or flue work to confirm safe exhaust of combustion gases.

When to call a professional

- Call a Gas Safe registered engineer if the C6 code returns after basic checks, if you find wiring damage, if the fan needs replacement, if electrical or PCB testing is required, or if you are unsure at any point. Do not attempt to modify gas or sealed components or perform live voltage tests yourself.

Final notes

- The boiler locks out to protect occupants; persistent C6 faults should be treated as urgent. A competent heating engineer will carry out safe diagnostic tests, replace the fan lead/connector or fan assembly, check the air pressure switch and flue, and correct any PCB or wiring faults. If you are not qualified to work on gas appliances or live electrical circuits, arrange a Gas Safe registered engineer to repair the fault.