Should I buy a combined smoke and CO detector or a CO-only unit?

Combination smoke-and-CO alarms save wall space and a sensor slot, and several in this roundup are well-rated multipacks. However, smoke and CO sensors age at different rates, so a combo unit must be replaced when either sensor expires. CO-only units like plug-in models are often cheaper to place near fuel-burning appliances and bedrooms where CO risk is highest.