Air conditioning and heat run off two completely separate systems that share one set of vents, which is why a car can blow ice cold in August and nothing warm in January. Knowing which system your problem lives in is most of the diagnosis.
The one thing worth understanding about AC
Air conditioning does not use refrigerant up. It is a sealed loop, and the charge that went in at the factory should still be in there years later.
So a system that is low has a leak. That is not a maintenance finding, it is a fault, and it is why a top-up is not a repair. It buys you a season at most, and while the refrigerant is leaking out it is taking the oil that lubricates the compressor with it. The expensive failure at the end of a series of top-ups is almost always the compressor.
Finding the leak is the actual work. Dye and a UV lamp, an electronic sniffer, or a nitrogen pressure test depending on how fast it is going. Then everything comes out, goes across a scale, and goes back in by weight rather than by gauge pressure, because a system charged by pressure alone is a guess.
R-134a and R-1234yf are not interchangeable
Somewhere around the middle of the last decade the industry changed refrigerants. Most vehicles built from the mid-1990s to the mid-2010s use R-134a; most built from roughly 2015 onward use R-1234yf. They need separate machines, separate oil and different fittings, and putting one into a system designed for the other contaminates it. The label under the hood tells you which you have.
Why it blows cold at speed and warm at a light
A very common complaint and usually not the AC itself. At a standstill the only thing pulling air through the condenser is the cooling fan. If the fan is not running, or is running at the wrong speed, the condenser cannot shed heat and the system loses its bite. Then you move, air flows through it, and the cold comes back.
So this symptom points at the cooling fan, its relay or its control, and sometimes at a condenser packed with road debris rather than at refrigerant at all.
Heat, which is a different system entirely
Your heat is waste engine heat, routed through a small radiator behind the dash called the heater core. Anything that stops that working falls into three groups.
- Not enough hot coolant. Low coolant level, an air pocket in the system, or a thermostat stuck open so the engine never reaches proper operating temperature. The tell for the thermostat is that the temperature gauge sits low and the heat is lukewarm at best, and it usually sets a code for coolant temperature below expected.
- A blocked heater core. Sediment from coolant that was left too long. The classic sign is one heater hose hot and the other cool, and often a sweet smell or a greasy film on the inside of the windshield if it has started to leak.
- A blend door problem. The flap that mixes hot and cold air has an actuator, and when it fails you commonly get heat on one side and cold on the other, or a clicking noise behind the dash. This is electrical and mechanical rather than anything to do with coolant.
Weak airflow, whatever the temperature
If the air is the right temperature but there is not much of it, start with the cabin air filter. It is the most commonly neglected filter on the car, it is usually behind the glovebox, and on a car that has never had one changed it can be genuinely blocked. That is a ten-minute item.
After that: a blower motor that has started to fail, or a blower resistor, which produces the very recognizable fault where you get airflow on the highest setting only.
Smells, and what they mean
- Musty when you first switch on. Moisture and growth on the evaporator. Common and treatable. Running the fan without AC for the last minute of a journey helps dry it out.
- Sweet, with a film on the glass. Coolant, which means the heater core is leaking. Worth acting on, because it is also putting coolant into the passenger footwell.
- Burning oil. Not the climate system. Something is leaking onto hot exhaust and the air intake is bringing the smell in.
What Michigan does to it
Two seasonal patterns worth naming. The first hot day in June is when leaks that developed unnoticed over winter announce themselves, and it is also the single busiest AC week of the year at every shop in the area, which is an argument for checking it in May.
The second is that road salt works on the condenser, which sits at the very front of the car behind the grille and takes everything the road throws at it. A condenser that has corroded through is a common Michigan failure and not something that happens in a dry state on the same timescale.
What a proper diagnosis looks like
Gauges on both sides of the system, because the relationship between high and low pressure is what tells you whether you are looking at a charge problem, a blockage, or a compressor that is not pumping. Vent temperature measured rather than judged. The cooling fan checked under command. Cabin filter, blower and blend doors checked before anybody talks about refrigerant, because those are cheaper and more common.
Then a straight answer. If your AC needs a cabin filter and a fan relay, that is what we will tell you, and it is not a big job. If it needs a compressor, you will get that in writing with what failed and why, because a compressor that failed for a reason will fail again if the reason is still there.