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).
6. Arrange a qualified inspection of fuel-burning appliances and venting before using them again.
7. Verify protection: install CO alarms on every level and near sleeping areas; test and replace as required.

Warning Signs That Suggest a CO Problem

A CO alarm is the clearest warning, but other clues can matter — especially when they happen together:

• 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.
• Soot/staining around an appliance, vent, or fireplace, or signs of poor draft/back-drafting.
• A pilot light that repeatedly goes out, unusual flame behavior, or strong exhaust where it shouldn't be.
• 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.

Beyond symptoms in people and pets, there are environmental carbon monoxide leak signs to watch for: soot or dark staining on walls, ceilings, or around appliance vents; unusual condensation on windows or cold surfaces near fuel-burning equipment; a stale, stuffy, or unusually warm atmosphere when heating equipment is running; and a pilot light that repeatedly goes out or a burner flame that appears yellow or orange instead of blue. None of these prove a CO leak on their own, but when you notice several together — especially alongside physical symptoms — they strengthen the case for immediate action: leave the building and call for professional help.

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 (every 30-60 seconds): usually means low battery or end-of-life. Replace the battery or the unit. Check the manual for your specific model.
• 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 suggest a problem with combustion or venting — but they are NOT a substitute for a working CO alarm. You can have dangerous CO levels with none of these visible signs present. The safest approach to detect carbon monoxide without detector backup is to treat any combination of unexplained symptoms and appliance problems as a reason to evacuate 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.

Apartment and Shared-Building Scenarios

In multi-unit buildings, CO can sometimes spread through shared walls, hallways, or venting systems — especially if there are shared mechanical rooms, boilers, fireplaces, or attached garages. If your CO alarm sounds, follow emergency steps first, then notify building management. Encourage neighbors to take alarms seriously and to evacuate if advised by responders.

In apartment buildings, CO can travel in ways that are not obvious. Shared ventilation shafts and ductwork can carry exhaust from one unit's malfunctioning appliance into neighboring apartments. Underground or enclosed parking garages attached to the building can allow vehicle exhaust to migrate into living spaces above — especially if garage ventilation is inadequate. Negative air pressure inside the building (caused by exhaust fans, dryers, or HVAC systems) can pull combustion gases backward through vents and flues, a problem known as back-drafting. If your CO alarm sounds but you don't have any fuel-burning appliances in your own unit, the source may be in a neighboring apartment, a shared mechanical room, or an attached garage. Report it to building management and to emergency services — the problem may affect more than just your unit.

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: Inspection and Prevention

After a CO alarm event or suspected exposure, don't simply reset the alarm and resume normal activity. Use this checklist to how to check for a carbon monoxide leak source and prevent recurrence:

1. Have a qualified technician inspect all fuel-burning appliances and venting with calibrated CO testing equipment.
2. Do not use the suspected appliance until it has been repaired or cleared by a professional.
3. Check that CO alarms are present on every level of the home and near sleeping areas — replace any units that are past their expiration date (typically 5-7 years).
4. Test all CO alarms after the incident to confirm they are functioning properly.
5. Schedule annual maintenance for furnaces, water heaters, fireplaces, and any other fuel-burning equipment.
6. Review ventilation — ensure vents, flues, and chimneys are clear and properly connected.
7. If the incident involved a generator or grill, review placement rules: outdoors only, well away from doors, windows, and vents.

For complete prevention guidance, see the prevention checklist page. For more on identifying the source, see 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: February 15, 2026