When I switch on the vents the cabin starts smelling

See above please

My car has 40000 miles.
My car has an automatic transmission.

Experienced mechanics share their insights in answering this question :
This is a common issue when the cabin filter needs to be replaced. Your vehicle has an air filter that filters the outside air as it is circulated into the vehicle when using the heater or air conditioning system. As the outside air is passes through your car’s system, it collects dirt, dust and other impurities much like your engine air filter does for the engine. When this gets dirty and collects dirt and debris from outside, this will tend to emit an odor like smell when the vents are opened as you have experienced. This is easily remedied by having this filter replaced with a fresh clean filter. I would recommend having an expert from YourMechanic come to your location to replace your cabin air filter.

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Just a note: If replacing your cabin air filter doesn`t eliminate that musty smell coming from the vents when the a/c is running, it is possible mold or mildew has built up inside your system`s evaporator.
A dirty cabin air filter can become a source of bad odors by trapping moisture and contaminants that promote bacterial growth. If your customer is complaining about a foul smell emanating from the interior of their car, it could be time for a “fresh” cabin air filter.
Fewer Odors

Mold can build up on the filter if you don`t change it, which can cause bad odors and smells. Mold is also extremely unhealthy to breathe in, as it can make a person very sick. But by replacing the cabin air filter regularly, you can avoid breathing in that unhealthy mold and keep your cabin smelling fresh.

Rattling Noise from the Air Vents

If the filter hasn`t been changed in a long time, debris can clog and collect in the filter. It`s possible to hear a rattling sound from leaves or debris collecting in the blower motor. This can happen if the cabin air filter was removed and not replaced.

This is completely normal. When using activated carbon filters, this is common and can often be expected due to the initial absorption of compounds in the filter. This is normal and not harmful in any way. The odor should subside within 1-3 weeks depending on the flow setting your units are running at.
When your air purifier`s pre-filter or filter surface (B indicated in above image) is dirty, or not cleaned regularly, it may cause a dusty odor or bad smell. You need to clean the pre-filter or filter surface once a month or when indicated by the appliance.
Not only will a new cabin air filter improve the quality of the air inside your vehicle, but it will also significantly improve the airflow from your vehicle`s heating and air conditioning system.
In those vehicles, a restricted cabin air filter will prevent good airflow in any mode of operation. The cabin air filter prevents the evaporator and heater core from becoming clogged with debris such as dirt, leaves, and bugs, which can result in restricted airflow, affecting the system`s efficiency.
If you do not opt for a trusted vendor, you might end up with low-quality spare parts which might decrease your driving performance. When it comes to the air filter, a bad air filter can make a funny and loud noise to disturb you while driving your car.
Technically, changing your cabin air filter might not make the air `colder` if you measure it with a thermometer. However, a fresh filter can significantly improve airflow, which affects how cold the air feels inside your vehicle.
The solution to most of your air purifier`s smells is as simple as cleaning or replacing the filters. If your air purifier contains a washable filter, then cleaning it up will be enough. Simply ensure that the filters are clean and that they are not the cause of the smell. Use non-corrosive cleaning agents only.
With a clogged cabin air filter, the lack of airflow can be felt at the dashboard vents and will result in lackluster defrosting and defogging performance as well as reduced air conditioning and heating functionality.
A dirty air filter can make your home smell musty and stale. A weaker airflow prevents air from adequately circulating through your home, letting bad smells from cooking or other activities linger longer.
Your car actually has two separate air filters: a cabin air filter and an engine air filter. Each of these filters works to keep harmful contaminants out of your vehicle.
Place baking soda into a jar and add essential oils. Make several holes in the top of the lid. Baking soda removes unwanted odors, while the essential oil gives your car a more enjoyable scent. Add essential oil to rice and place in a vented jar.
If the car`s air filter isn`t doing its job, then dirt can enter the engine and make its way into the oil supply over time. This can wreak havoc on expensive internal components of the engine, such as the valves and valve seats, piston rings and cylinder walls.
If a car is idling, the engine should give a typical hum. A clogged air filter limits air supplied to the combustion chamber. The combustion chamber, in turn, can sputter, pop, or give cough-like sounds. Odd sounds may also be accompanied by vibrating or shaking.
By installing your filter backwards, air will have a harder time flowing through the filter and your air handler will have to work harder to make up for the loss of airflow. This could lead to higher utility bills and possibly damage your furnace or air conditioner.
By replacing your filter you can improve airflow, thus lessening the load on the fan`s motor. As a result you can substantially lower your monthly heating costs. Improved Indoor Air Quality. Properly selected air filters help to improve your indoor air quality by trapping dust, allergens, mold and other pollutants.
Driving without an Air Filter

While there isn`t a direct impact on your engine`s performance with a broken or malfunctioning cabin air filter, it still isn`t the best idea to do it. Particularly if you are using your air conditioner or heater.

If your car`s air filter is very dirty and you have a bit more time on your hands, you can clean it with water. Cleaning an air filter with water takes a lot longer, especially when you factor in the drying time, but it will get you great results.
Fuel Efficiency

For example, if you drive an older model, studies show that changing a dirty air filter can increase fuel mileage as much as 14%. If you drive a newer model, you can expect a clean cabin air filter to increase acceleration by 6-11%.

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Fuel pump engages. Batery good, alternator good volts, jump start helps when wont start. Temp does not matter. Intermittent?
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Vehicle won’t start, once it’s jump started it only stays running until you turn it off, then it work start again without jumping.
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Cranks over start than dies
ANSWER : Hi There,
The idle air control valve is a small valve on the engine’s intake system that reads the air intake as it comes into the motor. This is controlled by the car’s ECM which uses this information to make adjustments to the air/fuel ratio depending on various inputs such as outside air temperature, intake air temperature, load and various other things. As you accelerate, your car’s engine is receiving a much higher dose of fuel than when at idle and conversely, when you let off the gas pedal, there is a sudden change in this fuel supply as a result of your foot letting off the pedal. When this happens, the job of the idle air control valve is to bring this deceleration down to a slow and smooth idle rather than suddenly cutting off the fuel supply causing the motor to die. When the idle air control valve is not working properly, this cause a disruption in this process resulting in the engine not being able to idle properly. I would recommend having a professional come to your location to diagnose and inspect your vehicle.

I started my car this morning and as I started down the road there was smoke coming out if the vents. The smell was like a wood fi
ANSWER : There are a few different things going on with your 1994 Chevrolet Cavalier. If smoke is coming from the vents the most likely cause is an electrical fire either inside the vehicle or from the engine bay, or smoke from burning oil in the engine bay. In either case the vehicle should not be driven and the root cause can be determined. Electrical fires can be caused by hidden wires rubbing together over time until the short and can be difficult to detect. A qualified Technician such as one from YourMechanic will be able to diagnose your electronic issue and ensure that your vehicle does not start smoking again.

Plastic burn smell when opening driver door. Cannot smell inside cabin. Oil full, brakes ok.
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72 Chevelle cranks but won’t start and has to be flooded but smell no gas
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Why does my car struggle to start every 4th to 5th time?
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Soap smell coming out of car after cabin change. Changed filter the the smell is still coming. What to do about this strong smell?
ANSWER : Hi There,
As you know, your car has a cabin filter that serves to filter the air that is introduced to the cabin inside the vehicle. The best way to remove your cars interior ac odor is to use a cleaner that uses enzymes. Enzymes in cleaners kill odors at the source and completely kill the smelly substance material and therefore stopping the a bad smell. To get rid of the smell, try the following: First turn on the ac to full, make sure the “recirculate” button is not on, spray a large amount of cleaner with odor killing enzymes into the EXTERIOR air intakes that are located just below the windshield wipers, let the car run for about 10 minutes with the ac on, after 10 minutes turn off ac and spray the cleaner into the ac vents on the INSIDE of your car. After doing all this, let the car sit for 20 minutes turned off and windows rolled up. Then repeat if the smell is still in the car. By doing this you are killing the bacteria or mold in the car ac vent ducts and also the evaporator mold that may be present.