A CO detector measures carbon monoxide gas in the air using an electrochemical sensor, and it alarms once CO reaches a set time-and-concentration threshold.
A CO alarm that stays silent at 40 ppm isn’t broken — it’s following its standard. Unlike smoke alarms, CO detectors wait, measuring how much CO is present and for how long, before deciding exposure has turned dangerous. That timing rule explains almost everything about how these devices behave.
Below is what the sensor measures, the three alarm test points UL 2034 uses, and how to read an alarm when it fires.
What Does a CO Detector Actually Detect?
A CO detector detects carbon monoxide (CO) gas in ambient air — nothing else. It does not measure carbon dioxide, smoke, natural gas leaks, or “all poisonous gases,” despite some product listings.
CO is a byproduct of incomplete combustion. Intertek documents referencing UL 2034 name common sources: exhaust from internal-combustion engines, abnormally running fuel-fired appliances, and fireplaces. Because CO is colorless and odorless, the sensor is the only way to know it’s there.
UL 2034 devices are also built to sound before CO levels cause loss of the ability to react — that margin is the point of the alarm.
How Does the Sensor Inside a CO Detector Work?
Most consumer CO alarms use an electrochemical sensor, which produces a small electrical current when CO enters its electrolyte-based cell. The detector reads that current to infer CO levels in surrounding air.
The National Institute of Standards and Technology describes this electrochemical design as typical for consumers, noting metal-oxide and color-changing-gel detectors also exist. Sensing is ambient-air based, so the reading depends heavily on where the unit sits relative to a CO source.
That’s why placement matters more than brand. A detector on the far side of a closed door from a faulty furnace reads lower than one nearby — not because it’s less sensitive, but because less CO reached it.
How Long Before a CO Detector Sounds?
A CO detector doesn’t sound at a fixed concentration — it uses time-and-concentration thresholds, so higher CO levels trigger faster. Under the UL 2034 test framework, that produces three alarm windows.
| CO Level | Alarm Window | What It Means |
|---|---|---|
| 70 ppm | Within 60 to 240 minutes | Low, sustained exposure |
| 150 ppm | Within 10 to 50 minutes | Moderate exposure building |
| 400 ppm | Within 4 to 15 minutes | Fast-rising, serious exposure |
| Below 30 ppm | No indication | Devices stay quiet by design |
Those windows come from UL 2034, Single and Multiple Station Carbon Monoxide Alarms, the standard most consumer units are listed to. Intertek’s summary of the standard and its 2024 edition confirms the 70, 150, and 400 ppm test points. Before deciding one alarm covers a garage, basement, and hallway, it’s worth knowing which units hold up under real conditions — a tested roundup of auto CO detector models worth comparing covers that ground.
Why Your Detector Has to Wait
The delay is intentional. Per UL 2034, a detector won’t indicate anything below 30 ppm and only displays CO levels once they matter, with display accuracy within about ±30% of actual concentration. A number on the screen is an estimate, not a lab measurement.
That design reduces false alarms from trace CO — a car idling outside, a briefly backdrafting water heater — without muting a genuine threat. Intertek notes the 2024 edition kept these display and threshold rules in place.
A CO alarm is also not a substitute for fixing a combustion appliance or ventilating an area. It warns you; it doesn’t solve the source.
What to Do When the Alarm Sounds
Treat a sounding CO alarm as an emergency: get people and pets outside into fresh air, and ventilate the space once everyone is clear.
Call 911 or your fire department from outside, and don’t re-enter until responders say the air is safe. If anyone has a headache, dizziness, nausea, or confusion, say so — those symptoms make it urgent.
Once everyone is safe, have a qualified technician inspect fuel-burning appliances and any attached garage before relying on the space again. A detector that alarmed correctly did its job; the next step belongs to a professional.
FAQs
Can a CO detector detect carbon dioxide?
No. A standard consumer carbon monoxide alarm senses CO gas only, with no element for carbon dioxide. Some devices combine a CO sensor with a separate CO2 monitor, but that’s two sensors in one housing, not one doing both jobs. Read the product label to confirm what’s inside.
Does a CO detector sound at a specific ppm?
No single number triggers it. UL 2034-listed alarms work on time-and-concentration thresholds: 70 ppm should alarm within 60 to 240 minutes, 150 ppm within 10 to 50 minutes, and 400 ppm within 4 to 15 minutes. Higher concentrations trigger faster alarms, while levels below 30 ppm produce no indication.
Will a CO detector work above 12,000 feet?
Many listed units specify operation up to roughly 12,000 feet above sea level, and performance can shift at higher altitudes. At high elevation, check the datasheet or manufacturer guidance for your model before assuming it will alarm on the same schedule as one at sea level.
References & Sources
- National Institute of Standards and Technology. “How Do Carbon Monoxide Detectors Work?” Describes electrochemical, metal-oxide, and color-changing-gel sensor types.
- Intertek. “SUN — UL 2034, Single and Multiple Station Carbon Monoxide Alarms.” Covers the UL 2034 standard, its 2024 edition, and display accuracy rules.
- U.S. Consumer Product Safety Commission. “CO Alarm Conformance Report Phase I.” Reports on how consumer CO alarms perform against UL 2034 requirements.
