Is it safe to warm up a car in a garage with the door open—when it’s an attached garage and kids are sleeping inside?

Short answer

Why people ask this

People with attached garages sometimes crack the garage door and idle the car to warm it up, assuming the open door vents exhaust safely. Parents often try this early in the morning while their kids are still asleep. The mix of an attached garage and a door-open idle feels convenient, but exhaust gases can still drift into the house. Cold starts also produce more carbon monoxide, and sleeping children won’t notice or report early symptoms.

When it might be safe

There are no commonly accepted situations where this is considered safe.

When it is not safe

• Attached garages share air pathways with the home; CO can enter through door gaps, ducts, and drywall penetrations even if the garage door is open.
• Cold-start idling produces high CO levels; the plume can pool in the garage and seep indoors as the furnace or bathroom fans create negative pressure.
• Sleeping children are more vulnerable: they won’t perceive headache or dizziness and may be affected at lower concentrations.
• Modern vehicles (including remote start) still emit CO; catalytic converters are less effective until fully warm, increasing early exhaust toxicity.
• Wind direction and temperature inversions can blow exhaust back into the garage and toward the interior door, despite an open exterior door.

Possible risks

• Carbon monoxide poisoning to occupants, with children at higher risk of hypoxia and long-term neurodevelopmental effects.
• CO accumulation in the home that can persist after the car is turned off, especially in tight, energy-efficient houses.
• Loss of consciousness during sleep with no early warning if CO alarms are missing, silenced, or malfunctioning.
• Secondary hazards such as starting the furnace or water heater drawing garage air into return ducts, spreading contaminants.

Safer alternatives

• Back the car completely out of the garage before starting it; warm it outdoors, several feet from the house and away from vents and doors.
• Use a block heater or battery/engine pre-heater on a timer to reduce cold-start emissions and the need to idle.
• Start driving gently after 30–60 seconds outdoors; use seat and steering-wheel heaters and defrosters to warm the cabin while moving.
• Pre-clear frost with windshield covers, scraper, or an electric defroster (engine off) before moving the vehicle outside to start it.
• Install and regularly test CO alarms in sleeping areas and near the door to the attached garage; keep the interior garage door well-sealed and self-closing.

Bottom line

Do not warm up a car in an attached garage, even with the door open—especially while children are sleeping. Move the car outside first, then start it, and rely on pre-heating tools and CO alarms for safety.

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