Can I Leave My Car Running for an Hour? Fuel Cost and CO Risk

You can physically leave a car running for an hour, but the main limit is where it is parked, not the engine. In the open air an hour of idling mostly burns fuel, roughly a quarter to a half gallon per DOE. In a garage, or with snow packed around the tailpipe, the same hour can poison the people nearby with carbon monoxide. This guide gives the fuel and cost numbers, the CO safety limits from CDC, and what to do instead.

Understanding Your Car’s Idling Behavior

Idling means the engine is running while the car is not moving. The engine still burns fuel and makes exhaust, but the trip meter shows 0 miles, so idling gets 0 miles per gallon. The fueleconomy.gov driving guide says idling can use a quarter to a half gallon of fuel per hour, depending on engine size and air conditioner use, and that restarting takes only about 10 seconds worth of fuel.

Argonne National Laboratory measured one real car, a 2011 Ford Fusion with a 2.5 L engine, at 0.279 cubic centimeters of fuel per second while idling. Over an hour that is about 1.0 liter, or roughly 0.27 gallon, which sits inside the DOE range. A larger engine, or the air conditioner running, pushes the number higher.

What One Hour of Idling Costs

Multiplying the DOE range by a few example fuel prices shows what an hour costs. Argonne’s idling poster adds that each gallon burned emits about 20 pounds of carbon dioxide.

Fuel price per gallon	Cost of 1 hour at 0.25 gallon	Cost of 1 hour at 0.5 gallon
$3.00	$0.75	$1.50
$4.00	$1.00	$2.00
$5.00	$1.25	$2.50

Carbon dioxide for the same hour works out to about 5 to 10 pounds. A daily habit adds up: one hour of idling every day for a year is 91 to 183 gallons at those rates, or about $365 to $730 at $4.00 a gallon. Canadian figures run higher. Natural Resources Canada says 10 minutes of idling burns 0.25 to 0.50 liter, which is 1.5 to 3 liters, or about 0.4 to 0.8 gallon, per hour.

The Downsides of Prolonged Idling

Fuel Inefficiency and Wasted Money

The money is the smallest cost of one hour, but it buys nothing. Argonne estimated that if every car in the United States idled just 6 minutes a day, about 3 billion gallons of fuel would be wasted each year. The DOE Consumer Guide to Reducing Vehicle Idling says idling for more than 10 seconds uses more fuel and produces more emissions than stopping and restarting.

Engine Wear and a Cold Catalytic Converter

The EPA says unnecessary idling of cars, trucks and school buses pollutes the air, wastes fuel and causes excess engine wear. Argonne’s testing adds a second effect: the catalytic converter, which cleans the exhaust, warms up much faster while the car is being driven than while it idles. A car that is idling after a cold start is therefore running with the emissions equipment cold. For how long an engine can idle before problems show up, see how long you can leave a car idle.

Myths That Keep People Idling

Common belief	What the sources say	Source
Restarting uses more fuel than idling	Restarting takes about 10 seconds worth of fuel; any shutdown longer than a minute saves money	DOE, fueleconomy.gov
The engine must warm up before driving	Modern vehicles can be driven within seconds; most manufacturers recommend driving off gently after about 30 seconds	DOE, fueleconomy.gov
Turning the engine off and on damages it	Today’s gasoline and diesel vehicles do not suffer damage by being turned on and off; starters and batteries are much more durable	DOE Consumer Guide
A catalytic converter warms up fine while idling	It warms up much faster when the car is driven	Argonne National Laboratory

One limit on that second row: DOE says the engine should not be put under extreme load until it reaches normal operating temperature, so drive gently at first rather than flooring it. Our guide on whether warming up your car is a myth covers the cold-start details.

Risk of Carbon Monoxide Poisoning

This is the part of the question that can hurt someone. CDC describes carbon monoxide (CO) as an odorless, colorless gas that kills without warning, and says CO is produced any time you burn fuel in a car or truck. More than 400 Americans die each year from unintentional CO poisoning not linked to fires, and more than 100,000 visit an emergency department.

• Never run your car inside a garage attached to a house, even with the garage door open. For a detached garage, always open the door to let in fresh air.
• Have a mechanic check the exhaust system every year, since a small exhaust leak can build up CO inside the car.
• If you drive an SUV or car with a tailgate, open the vents or windows when the tailgate is open, because with only the tailgate open, exhaust is pulled into the vehicle.
• Symptoms are flu-like: headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. People who are sleeping or under the influence can die before symptoms appear.

Snow turns an ordinary driveway into a hazard. A CDC report on the January 1996 blizzard describes CO poisonings in the Philadelphia and New York City areas among people sitting in idling cars whose exhaust pipes were blocked by snow. One man was found unconscious about an hour after telling his wife he would test whether the car would run, having made no attempt to remove the snow around it. CDC’s advice after heavy snowfall is to check that exhaust pipes are clear before starting the engine. Our explainer on how cars produce carbon monoxide has more background.

If You Feel Symptoms

Treatment of CO poisoning requires ending the exposure and oxygen therapy, according to the CDC report above. So shut the engine off, get everyone into fresh air at once, and call emergency services. Do not wait to see whether a headache passes, and do not sleep in a running car.

When Idling for an Hour Seems Necessary

Most of the reasons people give for an hour of idling have a lower-risk answer. DOE says that when you wait for passengers in moderate temperatures, turning the engine off makes sense, and its guide lists the real exceptions: waiting in traffic for safety, defrosting windows in winter, and an emissions inspection station that asks you to keep the engine at operating temperature.

Situation	What the sources recommend	Source
Waiting in a parked car	Turn the engine off if you will be stopped more than about 10 seconds and are not in traffic	DOE, AFDC
Cold morning	Drive off gently after about 30 seconds; the engine and cabin warm faster while driving	DOE, fueleconomy.gov
Frosted windshield	Idling to defrost is a listed exception; prevent frost instead, or scrape	DOE Consumer Guide
Remote start	Avoid it where you can; long remote warm-ups are still idling	AFDC, Argonne
Low battery worry	Drive at least 5 miles between start cycles to fully recharge the battery	DOE, fueleconomy.gov
Drive-through or school pickup	Turn the engine off or park; idling more than 10 seconds uses more fuel than a restart	DOE, Argonne

Cold Weather

Natural Resources Canada says idling for more than 30 seconds has no benefit for the vehicle, and that driving for a few minutes is the most efficient way to warm the engine, drivetrain and cabin. Remote starters invite long warm-ups; the Alternative Fuels Data Center advises avoiding them, and Argonne cautions against overdoing a 10-minute remote warm-up. Our comparison of remote start vs idling in winter goes through the trade-offs.

Defrosting the Windshield

Frost is the most common reason for a long idle, and the cheapest fix is stopping it forming. A cover that sits on the glass overnight means there is nothing to melt in the morning, which is the case the box above is built for; windshield cover vs car cover compares your options. If frost has already formed, scrape or brush it off and see how to defrost a car for the order of steps.

Charging the Battery

DOE’s stop-start guidance is to drive at least 5 miles between start cycles to fully recharge the battery and to limit accessory use while the engine is off. An hour of idling in the driveway is a poor substitute for a drive. For a battery that keeps running low, test it rather than idle it; our guide on whether a car battery charges while idling explains what the alternator does at idle.

Idling by Engine Type

Gasoline Engines

Argonne’s gasoline test car, the 2.5 L Fusion, burned a steady 0.279 cubic centimeters per second at idle. Argonne also found that if you turn a warm gasoline car off for as long as 15 minutes, the catalyst is ready to work again as soon as you restart. Larger engines burn more; Argonne’s Figure 2 shows idling fuel use rising with engine size, and accessory loads add to it.

Diesel Engines

Argonne tested a 2009 VW Jetta TDI and found its catalyst warms up more slowly than a gasoline car’s. Near 0 degrees F, a diesel’s catalyst may never reach working temperature while idling from a cold start, so Argonne advises using remote start sparingly with a diesel. Utah’s state guidance, as summarized by the Alternative Fuels Data Center, says diesel passenger vehicles should limit warm-up to the time the manufacturer recommends. Your owner’s manual is the authority here.

Hybrids and Electric Vehicles

Hybrids shut the engine off when stopped, and DOE’s hybrid overview lists automatic stop/start among the technologies most hybrids use. The Alternative Fuels Data Center notes that electric vehicles and stop-start cars eliminate idling at traffic stops and in queues.

Is It Illegal to Leave Your Car Running?

Sometimes. DOE lists Massachusetts, Maryland, New Hampshire, New Jersey, Vermont and Hawaii as places where unnecessary idling can be fined, plus parts of California, Colorado, New York, Ohio, Utah and other states. The Alternative Fuels Data Center’s summary of Utah says idling of any unattended vehicle is prohibited, with a penalty of up to $750 and/or up to 90 days imprisonment. Its Maryland entry says a vehicle engine may not run for more than five consecutive minutes while not in motion, with exceptions such as traffic conditions, heating or cooling equipment, and bringing the vehicle to the manufacturer’s recommended operating temperature.

These are summaries, not legal advice, and local rules change. Look up your own city and state before you leave a car running unattended.

When to Consult Your Owner’s Manual

The DOE Consumer Guide notes that current owner’s manuals often recommend avoiding idling. Look in yours for the warm-up advice, the remote-start limits and any notes about running the engine while parked. If the manual and this guide disagree for your specific vehicle, follow the manual. A vehicle that overheats, stalls or runs rough at idle needs a technician, not a longer idle.

What This Guide Does Not Cover

• Heavy trucks, buses and commercial vehicles, which have their own idling rules.
• Fuel burn for your exact vehicle; the owner’s manual or a mechanic can speak to that.
• Medical advice; anyone with possible CO exposure should get emergency care.

Keep reading

• Is It Dangerous to Leave Car Running While Fueling: Safety Risks Explained
• Can You Leave Your Car On While Pumping Gas? Fire Risk Facts
• Is It Okay to Leave Your Car Running? Risks Explained

Frequently Asked Questions (FAQ)

Is it bad to let my car idle for 30 minutes?

It wastes fuel and does nothing useful for a modern engine. At the DOE range of a quarter to a half gallon per hour, half an hour burns roughly an eighth to a quarter gallon while the car goes nowhere. The EPA also links unnecessary idling to excess engine wear. If you will be stopped for more than about a minute and you are not in traffic, turn the engine off.

Can I leave my car running in the winter to keep it warm?

DOE says most manufacturers recommend driving off gently after about 30 seconds, and that the engine and cabin warm faster while driving. Never do it in a closed garage, and clear snow from the tailpipe first, because blocked exhaust has caused CO poisonings in idling cars.

How long can I safely idle my car?

There is no single safe number. The engine itself can run for a long time, but fuel burn, exhaust and local rules all work against long idling, and CO risk depends on where the car is parked. DOE and Argonne point to about 10 seconds as the break-even against restarting. Check your owner’s manual and local rules.

Can idling for an hour drain my car battery?

DOE advises limiting electric accessory use and driving at least 5 miles between start cycles to fully recharge the battery, and an hour in the driveway is not a substitute for a drive. If the car struggles to start afterward, have the battery tested.

Is it illegal to leave your car running and unattended?

In some places, yes. Utah prohibits idling any unattended vehicle, and Maryland limits idling to five consecutive minutes with exceptions, per the Alternative Fuels Data Center. Rules differ by state and city, so check your local ordinance.

Is it safe to sleep in a car with the engine running?

No. CDC says people who are sleeping can die from CO poisoning before they have symptoms. Do not sleep in an idling car, especially in snow, in a garage or anywhere the exhaust can collect.

Bottom Line

An hour of idling costs about $0.75 to $2.50 in fuel at the example prices above, adds 5 to 10 pounds of CO2, and gets nothing done. The serious risk is carbon monoxide in a garage or with a blocked tailpipe. Turn the engine off when you stop, drive off gently after about 30 seconds in the cold, and keep exhaust moving freely outdoors.