Is it safe to warm up a car in an open-door garage when kids sleep above an attached garage?

Short answer

Why people ask this

Parents with children sleeping in a room over an attached garage worry about cold mornings and quick departures. They hope that opening the garage door makes warming the car safe enough. The concern is whether carbon monoxide and other exhaust gases still seep into the house, especially into upstairs bedrooms. People also wonder if a running car for “just a few minutes” is low risk at night or early morning.

When it might be safe

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

When it is not safe

• Garage-to-house air pathways (ceiling penetrations, shared framing, door gaps) let exhaust travel upward into rooms over the garage.
• Opening the garage door does not stop backflow, eddies, or pressure differences that pull fumes into the house.
• Bedrooms over the garage are at higher risk due to stack effect and shared ceilings/duct chases.
• Nighttime idling coincides with kids sleeping—reduced awareness and slower detection of symptoms.
• Modern engines still produce carbon monoxide when cold-started; brief idling can quickly elevate indoor levels.

Possible risks

• Carbon monoxide exposure in the bedroom above the garage leading to headache, nausea, or potentially fatal poisoning during sleep.
• Fumes infiltrating through recessed lights, plumbing/electrical penetrations, and imperfect drywall seams in the garage ceiling.
• HVAC return leakage or shared duct chases moving exhaust toward upstairs rooms, especially when the furnace/air handler cycles.
• Accumulation of CO in the garage that seeps into the house even after the car leaves, prolonging exposure while kids remain asleep.
• False sense of security from a partially open door or a brief warm-up, which may not prevent indoor concentration spikes.

Safer alternatives

• Always start the car outdoors in the open air; back the vehicle completely out before warming it.
• Use remote start only when the car is parked outside, or rely on seat/steering-wheel heaters and drive off gently to warm the engine.
• Install and maintain CO detectors on every level and specifically in/near the bedroom above the garage.
• Air-seal the garage/living-area boundary (garage ceiling drywall, penetrations, weatherstripping on the house-to-garage door).
• Use an engine block heater or battery/thermal preconditioning (for EVs) to reduce cold-start emissions and warm-up time.
• Keep the garage well-ventilated when vehicles have recently run, and avoid running small engines (mowers, generators) inside.

Bottom line

Do not warm up a car in an attached garage—even with the door open—when children sleep above it. Start and warm the car outside, improve air sealing, and use CO detectors to protect the bedrooms over the garage.

Related questions

Search something else