Heating and air conditioning systems are maintenance-free for the most part, so when something bad happens, it's seldom because the owner neglected to have it serviced. The systems just wear out over time.
That includes the refrigerant for the air conditioner. It's in a closed system, and if the refrigerant is low it's because of a leak, not because you forgot to take it in to be topped off.
The main things an owner can do to maintain the heating and air conditioning systems is make sure there's enough coolant in the radiator and that it has sufficient freezing, boiling and corrosion resistance, and to change the cabin air filter on a regular basis. Outside air passes through the cabin filter to the climate system, and if it's clogged because it hasn't been changed in years, less air will get through.
Here are some common climate system issues:
- If the air conditioner doesn't blow cold air or cold air only comes out of some vents, it's probably low on refrigerant because of a leak in the system. Topping off the refrigerant won't fix a leak. A repair shop can find the cause, and sometimes that includes adding refrigerant and dye to trace where the leak is.
- Leaks can come from hoses or seals that are easy to find and fix, from a faulty compressor or corrosion on the condenser (located in front of the radiator). Compressors can leak or break, and they may signal they're about to expire by making unusual noises.
- Air conditioning evaporators, which do most of the cooling of the air, can also fail and corrode. What's worse, they're usually behind the dashboard and thus expensive to replace because of all the labor involved.
- If little air is coming through the vents (hot or cold), then it could be because the blower motor for the fan, or a switch or resistor that controls fan speed, isn't working. Another cause might be that electronically controlled "blend doors" that direct air flow are stuck or broken.
- A lack of hot air when you turn on the heater could be caused by a worn out thermostat in the engine's cooling system that prevents warm coolant from circulating through the heater core. That's where the warm air comes from to make the cabin toasty.
- The heater core could be clogged by sediment that's built up over time (a possibility that increases with longer intervals between coolant flushes) and needs to be cleaned. The core could also be corroded from a small leak or accumulated moisture. A valve that allows coolant to flow to the heater core might also be stuck in the closed position. The heater core is usually mounted near the A/C evaporator behind the dashboard, so it's expensive to replace.