Snow Around the Exhaust Pipe: The Carbon Monoxide Check to Do Before You Start the Car
Start no winter engine until the tailpipe is clear of snow and the snow packed around the rear of the car is dug away. That is the whole rule, and every official source behind this guide states some version of it: the CDC puts it as checking the tailpipe before getting into the driver's seat, Montreal's public health direction adds the snow accumulated all around the car, and Vaughan Fire and Rescue Service adds that the exhaust pipe of a running vehicle must never be covered with snow. Everything below is the reasoning, the order of operations, and the exit decision if the rule was broken before it was known.
General information only. Carbon monoxide poisoning is a medical emergency: if symptoms appear, get to fresh air and call 911.
Why a blocked pipe is dangerous so fast
Vaughan Fire describes carbon monoxide as an invisible, tasteless, and odourless gas, produced when fuels such as gasoline burn incompletely, that can cause serious illness or death. There is no smell to warn anyone, which is why the gas carries its silent reputation. The exhaust system's job is to carry that gas out of the vehicle, where, as the Weather Network's summary of the guidance puts it, it can disperse in the atmosphere. A tailpipe plugged with snow defeats that path. The CDC's warning is direct: if the tailpipe is blocked, carbon monoxide can build up quickly inside the vehicle, poisoning anyone inside. Montreal's public health direction explains the mechanism one step further: exhaust gets trapped under the vehicle and inside the car, in concentrations that can quickly become fatal. The Weather Network also reports that vehicle emissions are the number one cause of carbon monoxide poisonings in Canada, which makes this unglamorous two minute check the highest value item in the entire snow-clearing routine.
The pre-start clearing procedure, in order
The common habit is to start the car first, for heat and defrost, and then clear snow around a running engine. The official guidance reverses that order, explicitly: do not leave the car running while clearing it.
- Engine off, key in pocket. No remote start, no push-button warm-up, not yet. The check below has to happen before the engine produces anything.
- Go to the rear of the car first, before the roof, before the windshield. Find the tailpipe and clear the snow and ice out of and away from its opening, completely.
- Dig out around the car, not just at the pipe. Montreal's direction is to remove the snow accumulated around the exhaust pipe and all around the car before starting the engine, because a clear pipe opening into a wall of packed snow still traps exhaust under the vehicle.
- Check what the plow and the wind added. A car that was clear at bedtime can be buried at the rear by morning plowing or drifting. The check is per start, not per storm.
- Only now start the engine. If the car will sit running while the remaining snow is cleared, the pipe stays visible and unobstructed the whole time.
What success looks like: the tailpipe opening is fully clear with open air behind it, the snow around and under the rear of the car is dug back, and the engine was started only after both were true. If any part of that cannot be verified, the engine stays off until it can.
The garage rule and the remote start trap
Two modern comforts work against this check. The first is the garage. Health Canada's recommendation, as reported in the Weather Network's guidance, is not to leave a vehicle idling in a garage regardless of whether the garage door is open, because carbon monoxide can still build up in the garage and enter the vehicle through its air system as it runs. Vaughan Fire, relaying National Fire Protection Association tips, states the same rule from the other side: do not run a vehicle indoors even if the garage doors are open, and if a vehicle needs warming, remove it from the garage immediately after starting it.
The second is remote start, which fires the engine from indoors, before anyone has walked around the car or seen its rear end. A remote start after an overnight snowfall is precisely a blind start: the one check that has to precede the engine is the one the fob skips. On a snow morning, the remote start waits until after step 3 of the procedure above. The car's cabin will be a few minutes colder. That is the entire cost of the trade. One related habit while the shovel is out: the same municipal guidance that covers tailpipes also covers the home's furnace, dryer, and fireplace vents, which deserve the same two minute check on the same walk.
If you are sitting in a running car while snow falls
Sometimes the car is shelter: a storm stop, a long wait, a breakdown. The sources behind this guide support a short protocol rather than a blanket ban, and it runs on re-checking, because falling and drifting snow rebuilds the blockage around a stationary car.
- Start from a verified clear pipe, dug out the same way as the procedure above, with open space behind the car.
- Re-check the rear of the car whenever snow is accumulating, and treat a pipe you can no longer see as a blocked pipe: engine off until it is dug clear again.
- Watch the passengers who cannot report. Montreal's public health direction flags children, pregnant women, and seniors as higher risk, and notes that young children and babies cannot always say what they are feeling. Their quietness is not evidence of safety.
- Keep the exhaust system itself on the maintenance list. Both the Montreal direction and the Weather Network guidance call for regular exhaust system inspection, especially on older cars, because a leaking system gives exhaust a second path into the cabin that no amount of snow clearing fixes.
Symptoms: the exit decision
Montreal's public health direction lists the first symptoms as headache, nausea, fatigue, and dizziness, progressing with continued exposure to vomiting, fainting, and even death, and notes the symptoms are easy to mistake for other illnesses. Vaughan Fire's list runs parallel: headaches, nausea and dizziness, burning eyes, confusion, drowsiness, and loss of consciousness, described as flu-like. Flu does not arrive for every occupant of a car at the same time. Headache plus dizziness in a running vehicle, after snow, is treated as carbon monoxide until proven otherwise.
The response is not diagnostic. Vaughan Fire's instruction is to move to a fresh air location outdoors, make sure everyone is accounted for, call for help from the fresh air location, and stay there until emergency personnel arrive, and its symptom guidance is to go outdoors and call 911 immediately. Nobody goes back into the car for belongings, nobody drives themselves anywhere, and the engine question answers itself later, with the car aired out and the pipe dug clear. The check that prevents all of it takes two minutes, at the rear of the car, before the key turns.
— Owen Park, Automotive Writer