Direct answer
Smoke and carbon monoxide are different hazards
Smoke alarms use optical, ionisation or other approved sensing methods to respond to smoke. Carbon-monoxide alarms use a sensor designed for the colourless toxic gas created by incomplete combustion.
A device should be identified by its label, standard, instructions and expiry information. Shape, sound or a single flashing light is not enough to identify its purpose.
Combined products
A combination alarm must list both functions
Some products combine smoke and CO detection in one housing. Check that the product is certified for each claimed function and that the location follows the instructions for both hazards.
A combination device can reduce the number of housings, but it does not remove requirements for appropriate coverage, interconnection, audibility, testing and replacement.
Placement
The correct location differs by sensor type
Smoke, heat and CO alarms respond to different conditions and may have different positioning rules. An unsuitable location can create delay, nuisance alarms or poor coverage.
For workplaces and shared premises, detection should follow the fire risk assessment and system design. Domestic alarm placement should follow applicable housing guidance and manufacturer instructions.
Testing and replacement
Use the test function and observe end-of-life information
Test alarms at the frequency stated by the manufacturer and relevant premises guidance. Record workplace system tests and act on low-battery, fault and end-of-life signals.
The test button checks selected electronics and sounder functions; it should not be treated as proof that a device is correctly located or indefinitely serviceable.