The only reliable way to detect carbon monoxide (CO) in your home is with a working CO detector, because the gas has no color, no odor, and no taste. You cannot sense it without one. Carbon monoxide poisoning sends tens of thousands of Americans to emergency rooms each year and is responsible for hundreds of deaths annually.

Knowing how detectors work, where to place them, and what symptoms signal exposure could save your life. This post covers everything homeowners need to know about detecting CO and keeping their household safe.

What Is Carbon Monoxide and Why Is It Dangerous?

Carbon monoxide is a gas produced whenever a fuel-burning appliance, engine, or fire does not get enough oxygen to burn cleanly. Because it has no color or smell, you have no way to know it is building up without a detector. At low concentrations, CO causes headaches, dizziness, and nausea. At high concentrations, it can cause loss of consciousness and death within minutes.

Because the symptoms resemble the flu, many people do not connect them to a CO problem until multiple household members are affected. A home inspection that includes indoor air quality testing can identify potential CO risks from appliances, attached garages, and fuel-burning equipment before they become an emergency.

How to Detect Carbon Monoxide Without a Detector

The short answer: you cannot, reliably. Carbon monoxide produces no sensory cues. The only behavioral clue is physical symptoms that improve when you leave the house, which means exposure is already occurring.

Signs that may indicate CO exposure include:

• A headache that starts shortly after entering a space and fades when you go outside
• Nausea or dizziness with no clear cause
• Confusion or multiple household members (including pets) feeling sick simultaneously

If everyone in your home feels sick and symptoms improve outdoors, leave immediately and call 911. Do not go back inside until emergency responders have cleared it. A functioning CO detector is the only genuinely reliable method of detection.

How Carbon Monoxide Detectors Work

CO detectors use one of three sensing technologies to identify carbon monoxide in the air and trigger an alarm before concentrations reach a dangerous level.

Types of Carbon Monoxide Detectors

1. Electrochemical detectors use a chemical reaction to generate an electrical current when CO is present. These are the most accurate and most common types in residential use today.
2. Metal oxide semiconductor detectors use a sensor chip that changes electrical resistance when it contacts CO. These tend to be less expensive but also less precise.
3. Biomimetic detectors use a gel that changes color when it absorbs CO, which can be useful in areas without power, but they require replacement after each trigger.

For most homeowners, an electrochemical detector from a recognized safety brand is the right choice. The EPA’s guidance on carbon monoxide detectors recommends choosing a model listed by Underwriters Laboratories (UL).

Where to Place Carbon Monoxide Detectors

Placement matters. CO is roughly the same weight as air, so it distributes fairly evenly throughout a room rather than sinking to the floor or rising to the ceiling. Install detectors:

• On every level of your home, including the basement
• In or directly outside every sleeping area, so the alarm wakes you at night
• Near attached garages, where vehicle exhaust can seep into living spaces
• Within 10 feet of gas appliances (furnaces, water heaters, dryers) per most local codes

Keep detectors away from fuel-burning appliances, cooking areas, and humid bathrooms, as these environments can cause false alarms or damage the sensor over time.

How to Test and Maintain Your CO Detector

Test your CO detector once a month by pressing and holding the test button until the alarm sounds. Replace the batteries annually, or whenever the detector chirps to signal a low battery. Most CO detectors have a lifespan of 5 to 7 years. After that, the sensor degrades, and the unit should be replaced even if it still appears to function.

Write the purchase date on the back of your detector with a marker so you know when to replace it.

Common Sources of Carbon Monoxide in the Home

CO enters the home whenever a fuel-burning device operates in an enclosed or poorly ventilated space. The most common sources include:

• Gas furnaces and boilers that are cracked, dirty, or inadequately vented
• Gas water heaters with blocked or damaged flue pipes
• Attached garages where a vehicle is left running, even briefly
• Gas stoves and ovens are used for extended periods in a closed kitchen
• Fireplaces and wood stoves with blocked or poorly maintained chimneys
• Portable generators operated indoors or in attached garages
• Gas dryers with disconnected or crushed vent hoses

Generators are among the most common causes of acute CO poisoning and should never be operated inside a home, garage, or near any window or door. The 5 steps to prepare for natural disasters resource from HomeTeam covers CO risks during outages. The fireplace safety and maintenance guide covers how a blocked damper or dirty chimney can trap CO indoors.

What to Do If Your Carbon Monoxide Alarm Goes Off

A CO alarm is not a drill. Treat every alarm as real until you know otherwise. Follow these steps:

1. Get everyone out of the building immediately, including pets. Do not stop to collect belongings.
2. Leave the door open as you exit to help ventilate the space.
3. Call 911 from outside the building. Do not go back inside.
4. Wait for emergency responders to test the space and confirm it is safe before re-entering.
5. Identify the source with a professional before turning appliances back on.

If the alarm sounds and no one is experiencing symptoms, you may open windows and doors to ventilate, but still contact your gas company or a licensed HVAC technician to inspect your equipment before assuming the alarm was a false positive.

Resetting the alarm without identifying the source is dangerous. A CO alarm that triggers repeatedly with no apparent cause needs professional investigation.

Related Questions to Explore

• Can you smell or see carbon monoxide? No. Carbon monoxide is completely colorless and odorless. You cannot detect it with your senses under any circumstances. The only way to know CO is present in your home is through a working detector or through symptoms of exposure, which, by that point, means you are already being affected. A HomeTeam home inspection flags whether CO detectors are installed and in the right locations throughout the home.
• What are the first signs of carbon monoxide poisoning? The earliest symptoms of CO poisoning are often a dull headache, dizziness, and mild nausea. Because these symptoms mirror the flu, many people do not connect them to CO until multiple family members become sick simultaneously or symptoms improve when they leave the home. If you suspect your home may have a CO source, HomeTeam’s indoor air quality testing can assess CO levels.
• Where is the best place to put a carbon monoxide detector? Place CO detectors on every level of your home, outside each sleeping area, and near the garage or any major gas appliances. CO distributes evenly throughout a room, so ceiling or wall mounting both work. If you are unsure whether your current detector placement is adequate, a HomeTeam inspector can evaluate your setup as part of a home inspection.
• How often should carbon monoxide detectors be replaced? Most CO detectors should be replaced every 5-7 years, regardless of whether they still appear to work. The sensors inside degrade over time and become less accurate. Check the manufacturer’s date on the back of your unit and replace it on schedule. When HomeTeam conducts an inspection, the report will note any detectors that are past their service life or missing from required locations.

When to Call a Professional

If your CO detector has triggered an alarm, call 911 first and leave the building. Once cleared by emergency personnel, contact a licensed HVAC technician or your gas utility to locate the source.

Call a professional when:

• Your detector shows any CO reading above zero, even without an alarm
• You notice symptoms of CO exposure that resolve when you leave the building
• Your heating system, water heater, or gas appliances have not been serviced in over a year
• A new appliance has been installed and not yet professionally commissioned

HomeTeam’s indoor air quality testing checks for carbon monoxide and radon as part of a broader air assessment. If you are buying a home, pairing this with a comprehensive home inspection gives you a complete picture of the property’s safety. Call (844) 969-0458 or schedule online to get started.

Conclusion

Detecting carbon monoxide starts with one action: installing a UL-listed CO detector on every level of your home, especially near sleeping areas. Because CO is invisible and odorless, no other method reliably protects you or your family.

Key takeaways:

• CO detectors are the only reliable way to detect this gas in your home
• Place detectors on every level, outside every sleeping area, and near gas appliances
• If the alarm sounds, get out immediately and call 911 before going back inside

If you are concerned about CO sources in your home or want a professional assessment of your air quality, HomeTeam can help. Schedule your indoor air quality inspection today for fast, trusted results.