Boiler casing or nearby walls show signs of soot or black staining, possible smell of exhaust gases.

Safety first: This is potentially life-threatening. Turn off the boiler and gas supply immediately, ventilate the area, and do not use the appliance. Install or check carbon monoxide alarms.

Homeowner checks:

• Do not attempt to clean or repair the boiler yourself.
• Evacuate the property if you feel unwell (headache, nausea) and call emergency services if CO poisoning is suspected.

Engineer actions:

• Inspect flue integrity and correct any disconnections or blockages.
• Check burner and heat exchanger for heavy contamination; clean or replace as necessary.
• Test combustion with a flue gas analyser and adjust gas valve settings.
• Replace any damaged combustion chamber seals.

The boiler must not be used until a Gas Safe engineer has declared it safe.