Carbon Monoxide in Your Car

Two identical cars sat in a Hokkaido snowdrift with their engines idling. In the one whose tailpipe stayed buried, cabin carbon monoxide passed 400 ppm after 16 minutes. The one with the snow cleared from around its exhaust registered almost nothing across the same run. Because CO has no smell, carbon monoxide in car symptoms get read as tiredness or travel sickness until two people in the same vehicle feel it at once, so the warning signs are worth knowing before you need them. Snow is the fastest route into a cabin. The other two that matter are a body seal cut open by aftermarket work, and an attached garage, which 31 states now treat as a carbon monoxide hazard in its own right.

Quick Safety Summary

• If you feel sick while driving, pull over where it is safe and get out into fresh air.
• Never idle a vehicle in an attached garage, CO can build up fast and seep into living areas.
• In winter, keep the tailpipe clear of snow; a blocked exhaust increases CO risk inside the cabin.
• A keyless car can idle in a garage without a sound, so confirm the engine is off before you go inside.
• Ask a mechanic to check the body seals as well as the exhaust if the car has aftermarket wiring or rear-end repair history.

How Fast a Buried Tailpipe Fills a Cabin

Most advice about snow and exhaust stops at "keep the tailpipe clear". The Japan Automobile Federation (JAF, 日本自動車連盟) measured what happens when you do not. At Santa Present Park in Asahikawa, Hokkaido, JAF parked two identical cars in snow with the engines running, put a gas detector in each cabin, and set the air conditioning to fresh-air intake. One car stayed buried. On the other, the snow around the exhaust was cleared away.

Cabin CO in the buried car rose from the start of the test. It passed 400 ppm at the 16-minute mark and reached 1,000 ppm six minutes later, which was the ceiling of the instrument rather than the ceiling of the car. For scale, a home CO alarm certified to UL 2034 is required to sound within 4 to 15 minutes at 400 ppm. The car with its exhaust area cleared registered almost no CO across the whole run.

JAF also tested the move most drivers reach for first. With the driver's window open about 5 cm, cabin CO was already in the 100 ppm range within five minutes, hit 400 ppm at roughly 40 minutes, and then climbed into the 800 ppm range. Cracking a window slowed the build-up without stopping it. Clearing the snow away from the tailpipe was the only variable in the test that worked.

The Explorers That Measured High Had Been Cut Into

NHTSA opened preliminary evaluation PE16-008 in July 2016 on 154 owner reports of exhaust odor inside the cabin of the Ford Explorer. The agency upgraded the file to engineering analysis EA17-002 in July 2017 and closed it in January 2023 without ordering a recall. By then the docket covered about 1.47 million vehicles from the 2011 to 2017 model years, including the Police Interceptor version many departments drive, and more than 6,500 complaints. Measured accurately, the agency concluded, cabin CO in those vehicles sits below accepted health standards.

NHTSA's exceptions are the part a buyer can act on. Every vehicle that did measure high had been altered after it left the factory. In police fleets the damage came from upfitting, where installers drill through the rear of the body for emergency lights and radios and leave the sealing system open behind them. Among consumer cars the agency traced the highest readings to sealing broken in a rear-end collision and never resealed during the repair. The leak path ran through the body seals rather than through the exhaust system.

That points a used-car buyer at a different question. A loud exhaust is not the signal here. Ask what has been cut through the floor or the rear panels: a towing harness, a reversing camera, an audio install, or bodywork after somebody hit the back of the car. Ford paid for the inspection and resealing on 2013 to 2017 Police Interceptor Utility vehicles under customer satisfaction program 17B25, with no mileage limit, and that program expired on March 31, 2019. No equivalent ever covered civilian Explorers, so on a private sale the seal check goes on your own service order.

The Garage Alone Triggers the Law in 31 States

Never run a vehicle in an attached garage, and that includes the minute with the door up while the windshield clears. CO reaches the living space through the connecting door and the wall cavities behind it.

State legislatures reached the same conclusion and wrote it into statute. Of the 51 US jurisdictions tracked in our state law dataset, 31 name an attached garage as a condition that triggers the CO alarm requirement on its own, with no fuel-burning appliance needed anywhere in the building. Washington states it about as plainly as a statute can: under RCW 19.27.530, an all-electric house still falls inside the requirement when it has an attached garage. The car is the combustion appliance in that scenario. Check CO detector laws by state for how your own jurisdiction handles it.

How Carbon Monoxide Can Happen in a Car

Cars produce CO as part of exhaust. Normally, exhaust is routed safely out of the vehicle, but several situations can create risk:

• Exhaust leaks under the vehicle that allow fumes to enter the cabin (especially at low speeds or with ventilation drawing air inside).
• Idling or running a vehicle in an enclosed or semi-enclosed space (garage, enclosed parking, repair bay).
• Wind patterns that push exhaust back toward the vehicle or into nearby openings.
• A tailpipe blocked by snow, ice, or debris, which can disrupt normal exhaust flow (winter risk).

Because CO has no smell, the absence of an odor does not mean the air is safe.

Symptoms and Warning Patterns

CO symptoms are often non-specific: headache, dizziness, nausea, unusual fatigue, and trouble concentrating. In a vehicle the pattern carries more information than the symptom does. Exposure shows up as something that starts or worsens while you are in the car, clears once you step into fresh air, and comes back on the same drives.

Two details sharpen that picture. If a second person in the car feels unwell at the same moment, or the dog goes flat on the back seat, stop filing it under motion sickness. And if the fog arrives only with the heater or the defroster running, say so when you book the inspection, because that points the mechanic at air being drawn in from under the car rather than at anything you ate.

High-Risk Scenarios

Keyless Ignition / "Car Left Running" Risk

Modern engines are quiet at idle, and a keyless car leaves no key in your hand to tell you it is off. People walk into the house believing they shut it down. Treat a car running in a garage as an emergency: shut it off if you can reach it in safety, put the garage door up, and get everyone out of the house.

Winter Snow and Blocked Tailpipes

Snow packs around a tailpipe in minutes while you wait for the cabin to warm or sit in stopped traffic. The JAF numbers above show where that goes. Keep a brush or a folding shovel in the car through winter, clear the exhaust area before you start the engine, and clear it again at intervals for as long as you are stranded and running the engine for heat.

Sleeping or Waiting in a Running Car

Sleeping in a running vehicle carries the same risk the buried-tailpipe test measured, and it removes the person who would have noticed. Parked against a snowbank, a garage wall or a loading dock, the exhaust has somewhere to collect. If you need heat while stranded, follow local emergency guidance and keep a downwind window open.

Long waits count too. School pickup queues and rest stops keep an engine running for long stretches, often against a wall or between other vehicles. Turn it off when you can, and park in the open when you cannot.

What to Do If You Suspect CO in Your Car

1. If you are driving, pull over as soon as you can do it in safety.
2. Turn the engine off and get everyone out into fresh air.
3. Check for severe symptoms: confusion, fainting, chest pain, trouble breathing. Call emergency services if you see any of them.
4. Do not drive on while symptoms continue or while you suspect a leak.
5. If you were idling in a garage, leave the garage and the house, and call for guidance if anyone feels unwell.
6. Book an inspection, describe the symptoms and when they appear, and ask for the body seals to be checked along with the exhaust.
7. Do not reproduce the problem by sitting in the car with the engine running.

Emergency response guide: What to do if you suspect a carbon monoxide leak.

Prevention Tips for Vehicles

• On a used car with accident history or aftermarket wiring, ask the shop to check body seals and floor penetrations, not only the exhaust joints. That is where NHTSA found the leaks.
• Book the exhaust inspection when the noise changes, when you feel new vibration through the floor, or when the cabin smells of exhaust. Rust holes, cracked gaskets and loose joints turn up at the same appointment.
• Carry a portable CO alarm if you drive winter routes where a closure could hold you for hours. Nothing else in the car will tell you the cabin is filling while you sit out a blizzard with the engine on, and the JAF curve above shows how little time that leaves you.
• In shared garages and loading bays, park away from doors, windows and air intakes, where a running engine feeds the building's ventilation.
• Keep the CO alarms at home in service. An attached garage puts vehicle exhaust on the same side of the wall as the bedrooms.

Full prevention checklist: Prevention of carbon monoxide poisoning.

Sources & References

• CDC, Carbon Monoxide Poisoning
• US CPSC, Carbon Monoxide Information Center
• JAF (日本自動車連盟, Japan Automobile Federation), snow-buried vehicle CO user test, in Japanese: the cabin CO measurements cited above
• NHTSA Office of Defects Investigation, PE16-008 opening resume, July 2016, Ford Explorer exhaust odor, for the 154 initial owner reports
• NHTSA Office of Defects Investigation, EA17-002 closing resume, January 2023, for the 1.47 million vehicle population, the complaint count and the finding that the highest cabin readings came from upfitting and rear-crash sealing damage
• US EPA, Carbon Monoxide's Impact on Indoor Air Quality, which lists attached-garage exhaust as an indoor CO source
• NHS, Carbon Monoxide Poisoning
• SilentShadow state CO detector law dataset, 51 jurisdictions, for the attached-garage trigger counts

Last updated: August 12, 2026