How to Detect a Carbon Monoxide Leak

Many people expect a gas leak to smell, but you cannot smell carbon monoxide at all. So how do you tell if there is carbon monoxide in your house? In most cases, the first sign of a carbon monoxide leak is either a detector going off or multiple people feeling flu-like symptoms in the same space. This guide explains the warning signs of a carbon monoxide (CO) leak, what different alarm signals mean, how professionals test for CO, and the safest next steps when you suspect a problem in a house or apartment.

Quick Safety Summary

• You cannot detect CO by smell, a working CO alarm is your primary early-warning tool.
• If a CO alarm sounds, leave to fresh air, call for help, and don't re-enter until cleared.
• Key clue: flu-like symptoms that improve outdoors and affect multiple people or pets simultaneously.
• A "chirp" often means low battery or end-of-life, check the manual, but never ignore an actual alarm signal.
• After any incident, have fuel-burning appliances and venting inspected before use.

Step-by-Step Checklist

Use this checklist if you suspect a carbon monoxide leak:

1. Go to fresh air immediately (everyone, including pets).
2. Call emergency services / your local fire department for guidance and air testing.
3. Do not re-enter the building until professionals confirm it's safe.
4. If it's safe to do so, stop using the suspected fuel-burning appliance (do not attempt repairs).
5. Get medical evaluation if anyone has significant symptoms (confusion, chest pain, fainting, trouble breathing).

What to do in the hours and days afterwards is set out under After the Incident below. The full emergency sequence, including how to handle an apartment building and what has to be true before anyone goes back in, lives on what to do if you suspect a carbon monoxide leak.

Warning Signs That Suggest a CO Problem

A CO alarm is the clearest warning. The clues below are about people and timing rather than about the building, and they matter most when several line up at once:

• People or pets feel unwell at the same time indoors (headache, dizziness, nausea, unusual fatigue).
• Symptoms improve when you leave the building and return when you come back.
• The issue appears when heat, hot water, or a fireplace is running, and eases when it's off.
• A generator, grill, or vehicle exhaust used in/near a garage, basement, or close to windows/doors.

None of these signs proves CO on its own. The safe response is to treat the situation seriously, leave to fresh air, and get professional testing and inspection. Signs you can see on the equipment itself are covered under Can You Detect CO Without a Detector?

CO Alarm vs Chirp vs Error

When a carbon monoxide detector is going off, the first question is usually: is this an actual alarm or something else? CO alarms can signal different conditions, and the pattern matters:

• Continuous or repeating loud alarm: treat as a potential emergency, evacuate and call for help.
• Periodic single chirp: low battery or end-of-life, and the interval tells you which. Manufacturers do not agree on the code, so read yours before you act, and our chirp pattern guide sets out what each brand means by it. Kidde uses a chirp every 60 seconds for low battery and every 30 seconds for end-of-life, which means changing the battery on a 30-second chirp leaves an expired sensor on the wall.
• Rapid chirping or error codes: may indicate a malfunction. Remove the unit from the bracket and check for an error indicator or message.

Important: even if you suspect a "false alarm," prioritize safety. If anyone in the space has symptoms, or if you have any doubt, leave to fresh air and call for guidance. CO levels can fluctuate, and an intermittent alarm may reflect a real but variable problem.

Can You Detect CO Without a Detector?

The honest answer: there is no reliable way to detect carbon monoxide without a detector. CO has no color, no smell, and no taste. However, some secondary indicators can suggest a combustion or venting problem that may involve CO:

• Soot or dark residue on walls near appliances or vents
• Excess condensation on windows, especially near fuel-burning equipment
• A yellow or orange burner flame instead of a clean blue one
• A pilot light that keeps going out
• Stale or stuffy air when heating equipment is running
• An unusual smell of exhaust or burning where it shouldn't be

These signs point to a problem with combustion or venting, and they are NOT a substitute for a working CO alarm. You can have dangerous CO levels with none of them present. Without an alarm, the safest rule is to treat any combination of unexplained symptoms and appliance problems as a reason to leave the building and call for professional testing. Then install CO alarms as your ongoing protection.

How Professionals Test for Carbon Monoxide

If you're wondering how to test for carbon monoxide in your home, professional evaluation is the most reliable approach. Fire departments and qualified technicians use calibrated electronic meters to measure CO concentrations in different rooms and near appliances. They also inspect combustion equipment and venting for back-drafting, blocked flues, cracked heat exchangers, or misadjusted burners.

Professional testing matters because CO levels can fluctuate throughout the day, a spot check with a consumer device may miss intermittent problems. After testing, technicians can identify the source and recommend repairs before you resume using the appliance.

If you live in a rental or apartment building, report the issue to the property manager and request qualified inspection of both your unit and any shared mechanical equipment.

What the Meter Reading Means

Responders read numbers off the meter and often say them out loud, which leaves you holding a figure with no scale attached to it. The scale is built into your own alarm. UL 2034, the US performance standard for residential CO alarms, sets the time-to-alarm windows a certified unit has to meet: it must sound at 70 ppm somewhere between 60 and 240 minutes, at 150 ppm between 10 and 50 minutes, and at 400 ppm between 4 and 15 minutes.

Read backwards, that tells you what a number implies. Readings in the tens of ppm sit where the standard tolerates an hour or more before an alarm has to respond, which is why alarms are built to ignore short excursions there rather than to chirp at every gas stove. Readings in the hundreds sit where the standard expects a response inside minutes. These are equipment specifications rather than personal thresholds, so use them to understand why a responder is treating a number the way they are, and follow the instruction you are given rather than your own arithmetic. More on the standard: CO standards and reference documents.

Why Your Alarm Said Nothing While the Meter Read Something

The standard has a second half that answers the question people ask next, and almost nobody publishes it. UL 2034 does not only say when a certified alarm has to sound. It says when it must stay silent: a listed unit is required not to alarm when exposed to a constant 30 ppm for thirty days. That is a design requirement rather than a defect, written so that alarms are not set off by ordinary air pollution or by a properly installed and maintained appliance doing what it does.

The consequence is worth being clear about, because it cuts both ways. Your alarm staying quiet is not evidence that the air is clean; it is evidence that the air has not reached the concentrations the standard makes it respond to. A meter reading in the tens with a silent alarm is the two devices agreeing, not one of them failing, and a technician who tells you the alarm is fine is right about the alarm and has not yet said anything about the appliance. If low readings keep turning up in the same room, the thing to pursue is the source, and the fact that the alarm has never sounded is not an argument against pursuing it.

One City in the Country Asks for the Number Instead of the Box

Follow that reasoning to its end and you arrive at a question the law almost never asks. Carbon monoxide requirements across the United States are written about a device: install one, keep it operable, put it here rather than there. Whether the appliance is actually burning cleanly is a different question, and we have read 51 state instruments and 93 municipal codes without finding anyone who asks it. With one exception.

Saint Paul, Minnesota asks for the reading. Sections 34.11 and 34.35 of its Legislative Code let the enforcement officer require an owner to produce current proof that any heating or space-heating facility has been serviced by a licensed contractor, and the code specifies that the proof "must include a carbon monoxide reading". Not a receipt, not a certificate that somebody attended: a number, taken at the appliance, by whoever did the work. It is the only place in our whole survey where a jurisdiction wants the measurement rather than the presence of an alarm.

That is worth knowing wherever you live, because it is a specification you can borrow. When you book the annual service, ask the technician for the combustion analyser reading in writing, at the appliance, before and after whatever they adjust. A contractor who does that routinely will not blink; one who cannot produce a number has told you something about the service you are buying. It also gives you the thing this page keeps circling back to, a baseline: next winter's reading only means something if you have last winter's to set it against.

Apartment and Shared-Building Scenarios

In a multi-unit building the source is often not in your unit. Shared ventilation shafts and ductwork carry exhaust from one apartment's failing appliance into the ones around it. An underground or enclosed parking garage feeds the living space above it when its own ventilation is undersized. And negative pressure from exhaust fans, dryers or the building's HVAC pulls combustion gases backward through flues, which is the back-drafting problem at building scale. An alarm sounding in an all-electric apartment is the clearest version of this: nothing in your unit can produce CO, so something outside it did.

Follow the emergency steps first, then tell building management and emergency services, because the problem rarely stops at your door. What the guides on this query never say is what to do when management does nothing, and that is where the answer stops being about detection and starts being about your jurisdiction. Put the report in writing rather than making it by phone: a dated written notice is what starts the landlord's cure period in the states that set one, and it is the document that decides who carries the liability if the next incident is worse. What the landlord then owes you, how long they have, and whether you can withhold rent or repair and deduct are set state by state, and we hold the answer for all 51 in the tenant remedy matrix behind carbon monoxide leak in an apartment.

Reducing Your Risk

Prevention is simpler than troubleshooting during an emergency. Use working CO alarms on every level and near sleeping areas, maintain fuel-burning appliances, and never operate generators or grills in enclosed or semi-enclosed spaces.

See: Prevention checklist.

After the Incident: Testing the Alarms

Resetting the alarm and carrying on is the common mistake here, because the alarm going quiet proves only that the concentration dropped. Two things belong to detection rather than to the clean-up.

• Test every CO alarm in the building afterwards, and replace any that is past its end-of-life date. That date is 5 to 10 years depending on the model rather than a single number for the category, and a unit made before August 2009 may reach the end of its sensor life without telling you.
• An alarm that sounded once and has been quiet since has cleared nothing. What the air holds now is a question for a technician with a calibrated meter, which is the same limit this page opens with.

The rest of the aftermath sits on what to do if you suspect a CO leak, which sets out the conditions to meet before anyone goes back inside. For prevention, see the prevention checklist; for finding the source, the sources page.

Sources & References

• US CPSC, Carbon Monoxide Information Center
• CDC, Carbon Monoxide Poisoning
• NFPA, Carbon Monoxide Safety
• NHS, Carbon Monoxide Poisoning

Last updated: August 12, 2026