Act on a CO alarm before troubleshooting, site it to Approved Document J and the model's instructions, and know what its self-test cannot prove.
A carbon monoxide alarm adds a warning. It does not maintain the appliance, test its flue or check there is enough combustion air, and a CO-only alarm cannot detect an escape of unburnt natural gas or LPG. Know what the device is for before you read anything into a signal, or into silence.
Respond before troubleshooting
Treat a full CO warning as a potential exposure, even if nobody feels unwell. Get everyone into fresh air and stop using the suspect appliance; only switch it off if safe. For a suspected gas-appliance incident in Great Britain, call 0800 111 999 from a safe location. Stay out until you are told it is safe to go back. Do not restart the boiler to see whether the alarm sounds again, and do not send anyone back in to refit a removed battery.
After suspected exposure, call 999 for breathing difficulty, sudden confusion, unconsciousness, weakness, or chest or muscle pain. For other possible CO symptoms, contact NHS 111 urgently. Do not ask an affected person to drive. Someone feeling better outdoors does not mean the building is safe, or that they do not need medical advice.
Decision check: everyone is outside, but one person is struggling to breathe. Call 999 for an ambulance immediately and explain the suspected exposure; do not wait for appliance diagnosis.
Place the sensor and the warning properly
In England, for a new or replacement fixed gas-burning appliance other than one used solely for cooking, Approved Document J calls for a CO alarm in the appliance's room. Place it 1–3 m horizontally from the appliance. Ceiling-mounted alarms stay at least 300 mm from every wall. For wall mounting, choose a position above doors and windows, as high as practicable while keeping at least 150 mm below the ceiling. Those clearances avoid the still air at the wall and ceiling junction; check the alarm model's instructions too.
Those figures are for the appliance's room. In other rooms, such as a bedroom with no fuel-burning appliance and no flue passing through, follow the alarm's own instructions: placement can differ, so one mounting height does not suit every room.
Check again after decorating or new furniture: a cabinet, curtain or extractor can make a good position unsuitable. Hearing the test sound through a cabinet does not mean room air reaches the sensor.
An audible alarm attracts attention without anyone looking at it, unlike a colour-change indicator, but it does not guarantee that every sleeper will wake. Consider the layout, closed doors and occupants' hearing needs.
Test, maintain and replace the device
Follow the alarm maker's test and replacement instructions. A button test can check the device, but it cannot inspect the burner, flue or air supply. Keep the appliance's planned servicing and safety checks. HSE strongly advises home owners to have appliances maintained regularly. A landlord also has a legal duty to maintain the appliances and flues they supply to tenants, and to arrange the separate annual gas safety check. A working alarm changes neither duty. The annual gas safety check and planned servicing are separate jobs: one visit can cover both if the engineer does both, but booking a service does not mean the legal check was done.
A CO warning, low power, a sensor fault and end of life can each sound different. Read the model's instructions: not every chirp means CO, but never write off a reported full warning as a battery problem.
Decision check: the replace-by date has passed but the button test works. Replace the unit as instructed. A fresh battery or more frequent testing does not renew its service life.