Your car’s heating and cooling system operates using a few basic principles and some pretty rudimentary physics and chemistry concepts. However, that same system is constructed out of so many components that are buried within the hood and bowels of the vehicle that it’s sometimes difficult to know what may actually be causing a malfunction or performance problem.
Because it is so difficult to see every little thing going on with your car’s heating or AC, the best way to diagnose a problem is to rely on your senses and trust your instincts.
Problems You Can Feel
There are many issues you may notice related to the system’s temperature and functionality. They are the least ambiguous to recognize as heating and cooling problems because they include things like the AC not blowing cool air when turned on or the heater not actually doing anything to warm the car when activated.
The most common reason that a car air conditioner in otherwise good shape would start to blow hot or temperate air that is clearly not cooled, is due to a lack of coolant. Most often this means Freon, or whatever refrigerant your compressor uses to evaporate heat, has depleted.
If your coolant was recently filled, revisit the shop that did the recharging to see if anything was overlooked. However, if you can say for certain that your level of coolant should be good, but you see that it now isn't, chances are you have an actual leak.
If you have a leak, simply recharging the AC system won’t fix anything. Most of the time, your problem lies in at least one of the three big components or your AC system: the compressor, the evaporator, or the condenser. It also has a small metal can containing desiccant, which should be changed whenever the system is opened. The components are in different locations in the car, so there really is no such thing as replacing the whole air conditioner as many people think they have after a major repair.
Depending on the vehicle, there are several feet of hard and flexible lines that can become damaged and cause a leak. The best thing to do now is to have it evacuated and recharged with the addition of dye to aid in finding and sealing the leak.
WARNING: It is especially important to deal with any electrical problems that may have occurred if there is a leak. Any fluid leak in your car's engine or electrical system can be dangerous and lead to larger problems.
On the other side of the coin is heating. A malfunctioning thermostat could be the reason that your car is not heating up.
For example, if you’re driving on a particularly cold day and the temperature display in your dashboard claims the motor hasn’t reached its operating temperature, it means your thermostat is stuck open. You can further test for this issue by seeing if the ventilation blows heat when the car is idling but not when you’re actually driving.
By opening and closing, the thermostat helps keep the engine at correct operating temperature. When it goes bad, it will either stick open or closed. If it sticks open, your engine will be slow to warm up and won't stay at operating temperature. If it were to stick closed, the car would overheat because the coolant can't flow to the radiator.
Thankfully thermostats are both cheap to replace and easy to access physically within the vehicle.
Another issue that could leave you without car heat during the winter months is the heater core, even if you’ve already flushed it out. Since turning the heater on removes heat from the cooling system, it actually cools the engine. Often, heater cores plug up again even after they are flushed. Flush it again; if it goes away, great. However, it may only be temporary. If it’s being particularly stubborn, there are products that claim to help flush cooling systems that you may want to try.
Problems You Can Hear
Even when your vents are actually blowing cold air properly, if you hear something like repeated or intermittent clicking, it could indicate a problem.
The air conditioner dries, as well as cools the air. This allows it to absorb the moisture (fog) as it blows over the glass and clears the windshield. The clicking sound is a magnetic clutch, which allows the compressor to run or be turned off. The high or low limit switches usually cause the cycling of the compressor. These temporarily shut off the compressor to prevent damage to it.
If this clicking and stopping occurs when the defroster is used as well, your problem is likely related to low pressure in the component. You should take it to a specialist and get a full diagnosis.
Problems You Can See
These symptoms are easy to spot, as they usually affect what you see and do in the car’s cabin, but the solutions aren’t always straightforward
If your controls are unresponsive and won’t switch between vent or defrost, for example, but the system is still functioning and blowing cool air, you need to find a way to reset the climate control head. It involves pressing and holding certain buttons, though the specifics vary from vehicle to vehicle. Try calling the local car dealer for details or consulting an online manual now that you have a specific goal.
TIP: If you’re concerned that a local dealer won’t want to give you such technical information over the phone when you have no intent of buying from them, tell them you are a tech from another local dealer working on a used car.
HVAC Control Speeds
If the system is functioning but your HVAC control speeds aren’t working, or if you’re unable to cycle between the “1” and “2” settings, it means your resistor stack is bad. The blower motor resistor is usually located somewhere in the airflow ducting to keep it cooled preventing burnout.
If your air conditioner stops working and you’ve already checked the compressor and recharged the coolant, it’s possible you may notice a colored discharge, sometimes blue or green. Unfortunately, this means you are leaking refrigerant from somewhere.
Do not be alarmed by the color. It’s likely just a UV dye that has been intentionally added so that you could detect leaks more easily.
Alternatively, you may witness strange things being discharged inside the vehicle as well. If your vents ever start to blow out a white mist or vapor, check the vehicle’s undercarriage after you turn off the car from your drive. The mist you’re seeing is water vapor, and if the under carriage is dripping water, what you’re witnessing coming from the vent is essentially the cooling system making its own miniature clouds.
If there is no water dripping underneath, then it means your drain tube is clogged and condensate is accumulating in the heating and air box. If you unclog the tube, the clouds should go away.
Problems You Can Smell
Bad Odor from Vents
An air conditioner giving off a stench of syrup or hot antifreeze should be a tip that there’s either mildew on the evaporator or something wrong with your heater core. An additional test you can run to be sure is to see if your windows fog up especially easy while you’re driving at night. Quick-fogging windows coupled with the bad smell mean it’s a problem with the heater core.
They make a few chemicals that can be shot in there. A spray of bleach will work, also. The hardest thing is getting access to the core. You may have to cut a one-inch hole in the housing to get a good shot at it. Just make sure you know where you're drilling and stay well clear of the core. A little AC tape will cover the hole afterwards. Once you kill the smell, you can prevent further mold by turning off the AC and having the selector set on fresh air, not recirculation, with the blower on high. Let it run for a minute or two. This will dry the evaporator and ducts.
Gasoline Smell Coming Out of AC Vents
Gasoline smells coming from your car’s heating and cooling vents could mean there is a fuel leak somewhere under the hood.
WARNING: This is a dangerous situation, and you should have checked out by a mechanic immediately. If possible, consider having the vehicle towed to the mechanic instead of driving it there.