Answer first, context after
Do I need a carbon monoxide detector if I have a gas furnace?
Yes, and in California it is not optional: the Carbon Monoxide Poisoning Prevention Act has required CO alarms in every home with a gas appliance or attached garage since 2011, placed outside each sleeping area. A gas furnace burning correctly makes almost no CO. The alarm exists for the day something stops being correct, and CO gives no other warning.
Carbon monoxide is the one furnace topic where we drop the wry tone. It is colorless, odorless, and the symptoms of low-level exposure, headache, fatigue, nausea, read like an ordinary bad day, which is exactly what makes it dangerous in winter when the windows stay shut and the furnace runs all night.
The legal answer first: California’s Carbon Monoxide Poisoning Prevention Act, on the books since 2010, requires CO alarms in every dwelling with a fossil fuel burning appliance, a fireplace, or an attached garage. Single-family homes have been covered since July 2011. The law calls for alarms outside each sleeping area, and devices must be approved by the State Fire Marshal, which in practice means buying a listed brand from a normal retailer and checking the box for the certification mark. If you have a gas furnace, water heater, stove, or an attached garage, this law is about your house.
The practical answer: one alarm in the hallway outside the bedrooms is the legal minimum, and adding one near the furnace closet is cheap insurance. Check the date on the back: CO sensors wear out, and most alarms have a stated lifespan of several years, printed right on the unit. An alarm from 2015 guarding your hallway is furniture.
What a furnace has to do with it: a healthy gas furnace burns clean and vents its exhaust outside, producing next to no CO in your living space. The risks are a cracked heat exchanger, a blocked or disconnected flue, or a furnace starved of combustion air, which is why every furnace visit we make includes a combustion safety check and CO measurement, not as an upsell but as the part of the job that is not allowed to be optional. It is also half the argument for the pre-winter heating check: the failure modes that produce CO announce themselves to instruments long before they announce themselves to people.
If your CO alarm goes off: treat it as real. Get everyone outside into fresh air, call 911 from outside, and let the fire department clear the house before anyone goes back in. The furnace conversation with us comes after, and an after-hours call about a CO alarm is one we take at any hour without complaint.
Still stuck? That's what the truck is for.
The diagnostic is $89, waived when you book the repair, with a written flat price before any work starts.