How to Troubleshoot Your Car’s Air Conditioner and Diagnose Air Conditioner Issues

May 24, 2025

Our Gearhead Guru Guide series is designed to help you gain the knowledge and skills needed to care for your vehicle inside and out. We want to make regular maintenance and basic troubleshooting as easy as possible by sharing our wealth of knowledge! From washing and detailing your car, to organizing your garage, to preparing your vehicle for the ultimate road trip, we’ll be covering a little bit of everything.

Today’s topic is focused on your vehicle’s HVAC system, and how to diagnose and troubleshoot any issues that may arise. Even with regular maintenance, your vehicle’s AC is bound to break down or need repairs. Especially when the weather warms up and it’s used more frequently.

Let’s take a look at some of the basics of how the AC system works, and key terms you need to know!

A/C compressor

An engine-driven pump in the air conditioning system that increases the pressure of the refrigerant.

A/C drain hose

A hose used to drain condensation from the air conditioner evaporator.

Air conditioner refrigerant

The liquid used in air conditioning systems, which has the property of boiling at ambient air temperatures.

Air conditioning

The cooling and dehumidification of a vehicle’s cabin air.

Air conditioning filter

A device that removes contaminants from external air entering the ventilation system.

Air filter

A device that filters incoming air fed to the engine.

Climate control

The computer-controlled regulation of heating and air conditioning systems to accommodate a user-specified temperature setting.

HVAC

Heating, ventilation and air conditioning.

Radiator core & AC condenser

The part of a vehicle’s radiator and air conditioning systems made up of small-diameter tubes with a series of thin, heat-conducting fins in between.

Rear HVAC

The climate-control system used to regulate heating and air conditioning for the area of a vehicle behind the front row seating.

Your vehicle’s climate control system is a complex combination of components that include the compressor, condenser, receiver/dryer or an accumulator, a thermal expansion valve or orifice tube, and an evaporator. The compressor helps pressurize refrigerant to cool the air, and can sense the temperatures both inside and outside the vehicle, moves air to the condenser, etc. The condenser helps reduce the temperature and the pressure of any hot gasses emitted from the refrigeration system, and moves cold liquid refrigerant to your receiver/dryer or the accumulator.

If you notice that the air coming out of your car’s vents is not as crisp and fresh as it usually is, there may be trouble brewing. The most common parts to need service are the compressor and the condenser, followed by electrical shorts and broken fans. Additionally, low refrigerant (can be caused by leaks), a worn or broken v-belt or serpentine belt (helps the compressor function), or a malfunctioning compressor switch can sometimes also be at fault.

Let’s learn more about how to gain a better understanding of what’s happening to diagnose the issue before you bring it in to your trusted technician.

First, we’ll do a quick visual once over. Then turn your car on, put the temperature dial to the coldest setting on a high fan. Observe if the system is producing any air, how cold it is, if there are any odors. Listen closely to assess if there is reasonable air flow and no mechanical or other abnormal noises are present. When you bring the vehicle into the service center, this will help your mechanic or technician troubleshoot further. Let’s take a look at some of the other clues that can point you in the right direction for a diagnosis.

If the air comes out of the vents cool but not quite cold enough…this may indicate you need a recharge of your AC refrigerant. If your technician checks the refrigerant and it’s at an adequate level ,it may be a sign that you have a blocked or failed condenser unit; the condenser may have a blockage or be damaged, reducing or stopping its ability to convert the refrigerant from gas to liquid, leading to inadequate temperatures. It may also be a result of a failed clutch switch, rendering the air compressor inoperable, and preventing it from condensing liquid refrigerant into gas.

If the vents are producing tepid air and it isn’t cold at all…This is a sign that you may have a refrigerant leak in your system. A leak can happen at any time, and is most often caused by a puncture hole or broken hose, a malfunctioning compressor or condenser, or even the evaporator. You’ll need to take the vehicle to your trusted service technician; they’ll perform a special test utilizing a black light to look for the location of the refrigerant leak. Once they find the location of the leak, they’ll recharge the refrigerant and it should blow cold air again.

If your AC smells stale and mildewy….You’ll need to replace your air filter. As you drive, the air filter collects dirt, dust, water, and other pollutants and particulate matter. After some time, this can lead to build up and produce a stale odor. You can generally remedy this by replacing the air filter! If that doesn’t alleviate the problem, you can ask about antibacterial cleaning treatments. You should be checking and cleaning your air filter out at least every 3 to 6 months, with a replacement every 10-12 months.

If there is no air coming out of the vents at all…it’s time to take the vehicle into a trusted service technician. In general, no air flow from the AC can point to a much larger problem such as a blown fuse or bad relay, blocked air intake (either at the exterior windshield vents or in the recirculation process), damaged belts or detached belts, broken or blocked hoses preventing airflow from the vents, damage to the blower motor or blower resistor. If you’d like to try troubleshooting at home, you can replace the fuses or the relay yourself with a little elbow grease. However, if the problem persists your mechanic will administer a test to the other components and replace them as needed.

If your AC system makes a terrible noise when you turn it on… indicates a larger component in the system has started to malfunction or debris is blocking the airflow.. The system should be virtually silent with very little sound minus the flow of air from the vents. If you hear anything like rattling, banging, squealing, or other unusual noises , book a service appointment ASAP and avoid using the AC until you do. Your mechanic will perform a very thorough inspection to determine exactly which components are failing, blocked, or damaged and replace them.