This post was originally published on Reddit.

The following is based on official service documentation, personal experience and research driven by desire to know every little bit about this first car of mine. Sharing this because it may be interesting if this is your first car, but it may explain things in general. This post is specific to US MY2016+ models.

TL;DR: Air in the car generally:

• is sucked in through the vents from under the windshield,
• into the compartment behind the glove box,
• through the filter which you should regularly replace,
• then blower motor turbine,
• through an “Evaporator Core” if AC is present,
• a “Heater Core”,
• the vents into the main cabin,
• and goes outside through the pressure valve in the trunk area.

Now, details.

Starting from the front, the vents for air intake are under the windshield. They should be clean of the leaves and other objects as the HVAC system will not run properly if the airflow is restricted. When, however, something does fall into these vents, a Cabin Air Filter will stop it.

This filter (GM# 42666227 at the time of writing) should be replaced regularly, the manual suggests every two years or 22,500 miles max. You get dust, sand, leaves, twigs, insects, and other debris sucked in, which slowly clogs the filter pores. Filter MSRP is ~$24 and easy to replace, provided you have the right 7mm hex socket (9/32" is close enough) screwdriver and about half an hour to fiddle with the bolts.

Note, that it is possible to get a different filter, say Bosch 6039C HEPA filter, but my experience with that (apart from driving for a month with a strong chemical odor) is that due to increased density of the filtering material, blower motor struggles to push the air, and AC became less efficient. It does filter better, but I reverted to the OEM version.

Both AC Evaporator Core and Heater Core sit in the middle of the dashboard near the floor, that is why you hear liquid noises coming from inside the dashboard.

AC Evaporator gets liquid refrigerant from an AC compressor installed in the engine compartment on the passenger side. The latter drives the AC compressor by a belt, so AC will not work if the engine is not running. AC compressor then connects to a Condenser which sits at the bottom of the main engine cooling radiator and cools down the refrigerant that has just taken heat out of the passenger cabin.

AC Evaporator both cools down the air and removes moisture due to condensation (vapor turns into liquid when it is in contact with colder surface), water then drips outside through a drain hose on the left side of the passenger foot well. That hose goes into the engine compartment and drips water under the car. Since the AC compressor also becomes cold due to cold refrigerant, condensed atmospheric water will also be dripping down the compressor itself.

This is where the musky odor comes from – since the AC Evaporator Core becomes wet due to condensation, bacteria grows on the tubes and fins and may produce various unpleasant smells. Official manual provides a way of cleaning – taking apart the whole system and wash the AC Evaporator Core in vinegar and water, while a Service Bulletin (superseded by 99-01-39-004I, but has similar instructions) suggests dealerships to use a special cleaner to cover the coil with a "coil coating", or enable the “Afterblow” mode. I was not able to confirm the presence of the feature in MY2016 spark, so a separate module installation is needed if you want that feature - see the bulletin. It is the same process as turning off AC and leaving the blower on for some time (or until the from the vents stops being moist) to dry the evaporator core and limit the amount of moisture bacteria uses to grow.

Heater Core on the other hand is part of the Engine cooling system, circulating the engine coolant which is hot when the engine is hot. There is no auxiliary heating system in Sparks, so when set to “heat”, the air will be as cold as outside until the engine heats up. If your model has heated seats and steering wheel, I envy you 😊- those things are powered by electrical system and will work regardless of the engine temperature.

AC and the heater can run together to remove moisture from the air and heat it back up to some comfortable temperature, which is exactly how defogging setting works. There is nothing wrong about running the AC and heater at the same time when you want dry and warm cabin air.

The air can run in recirculated mode which is enabled by pressing the blower speed button (marked as ), in this case a valve located above the filter flips to a different position, sucking the air from behind the dashboard on the passenger side. This allows to rapidly heat or cool down the cabin or minimize amount of odor or smoke that’s coming inside the car from the outside.

The pressure valve in the rear center of the trunk under the latch and provides a path for the air to leave back to the outside world.

This concludes the post and I hope provides insight into how this system works in general. If you find something to be wrong, let me know.