Why Isn’t My Car AC Blowing Cold Air?

You're stuck in traffic on a hot day and warm air is coming from the vents. Not good. Here's why your air conditioner isn't working, and what you can do to fix it.

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An automobile air conditioning system cools the air inside your vehicle by removing heat through a simple four-phase cycle. Although an automotive AC system has few moving parts, diagnosing why your car is blowing warm air can be tricky. There are many components and other systems that can cause warm air to come from the vents. Here are the common causes an AC system is not blowing cold air.

Plus, check out these ten car problems you can easily fix yourself.

Refrigerant Leaks

Low refrigerant levels due to a leaky O-ring, seal or other component are the most common problems associated with a poorly performing AC system. Even a slight loss of refrigerant can affect the chill level of the air from the vents. A sure sign of low refrigerant is the AC compressor repeatedly clicking off and on, called short-cycling.

Locating refrigerant leaks can be difficult even for the pros. Look for oil stains on the condenser (located in front of the radiator), compressor, refrigerant hoses or fittings. Also, check that the AC service port (Schrader) valve caps and O-rings are in place and only hand-tight.

There are several DIY AC system recharging kits available. Some include oil; dye to help locate leaks; or additives that rejuvenate O-rings and seals. Get a kit that matches the type of refrigerant in your vehicle and includes a gauge to test system pressure before adding refrigerant. Recharging the AC system is usually a one-time fix. If your AC needs frequent recharging, it’s time to visit your repair shop.

Safety First!

Always wear safety glasses and gloves when working with refrigerant. Refrigerant freezes skin quickly and that is really painful. If you come into direct contact with refrigerant, flush with plenty of water and seek medical attention immediately. Read and follow all instructions that are included with your AC recharge kit and read and heed all warnings carefully. Plus, learn about the freon phaseout (and why you should care).

AC Compressor

The compressor is another common cause of auto AC problems. Check to make sure the clutch assembly — located at the front end of the AC compressor — is engaging.

With the engine running, blower fan on max high and AC controls set to the lowest temperature, the clutch should be spinning. You may hear a slight “click” or change in engine speed when the clutch engages. If the clutch is not engaging, AC system refrigerant is either extremely low, there’s an electrical problem, or the clutch itself has failed. Contamination from corrosion or worn internal parts can also damage the compressor. Compressor repairs are not a DIY fix.

AC Condenser

Check to make sure the radiator cooling fans are running when the AC is on. Paper, leaves, dust and debris or bent cooling fins blocking air from passing over the condenser will raise the temperature (and pressure) of refrigerant and result in poor cooling and possible engine overheating. Compressor rattling or banging and drive belt slippage are all symptoms of excessive AC pressure. Use your garden hose to flush the condenser and radiator fins.

AC Accumulator/Receiver Drier

These devices use desiccant (like those little packets you find in a box of new shoes) to absorb moisture in the AC system. Moisture can build up in a system that is leaking and low on refrigerant. Moisture reacts with refrigerant oil to form damaging corrosive acids. Worse, once desiccant becomes oversaturated it can seep into the AC system, damaging or clogging every AC component. That results in no cooling and the need for extensive repairs.

Expansion Valve/Orifice Tube

Modern AC systems use an expansion valve or orifice tube to control the amount of refrigerant entering the evaporator. Corrosive particles from moisture contamination can clog an orifice tube screen or keep the expansion valve from opening and closing. Out-of-spec AC system pressures can help determine if an expansion valve or orifice tube is causing AC problems.

AC Evaporator

If you can hear the blower fan running but there is barely any air coming from the vents, the evaporator fins are probably clogged with leaves or debris. If you notice a strange odor when using the AC, it usually indicates the evaporator is leaking refrigerant. Because the evaporator is usually tucked up under the instrument panel, your mechanic will have to figure out if the evaporator is the culprit.

Electrical Issues

On modern cars, there are any number of fuses, pressure/temperature sensors or switches that can keep the AC from turning on. A defective blower fan, engine cooling fans and relays can affect AC system operation and cooling performance.

You can test the fuses using an inexpensive 12V test light or digital voltmeter (preferred) and check that all electrical connections are clean, tight and secure. Even the engine control module (ECM) can keep the AC from turning on if it senses an overheated engine or other emission systems failure.

Clogged Cabin Filter

This one is often overlooked when diagnosing an AC system problem. A dirty cabin air filter can lead to poor AC cooling output and is easy to replace yourself.

Prevention and Cures

Running the AC 10 minutes a week, even in the winter, helps circulate refrigerant oil and lubricate the compressor and condition O-rings and seals. Take your vehicle to a certified technician if the DIY suggestions given here didn’t do the trick, or if the system won’t turn on at all. They have the specialty tools and equipment to accurately diagnose the problem, safely reclaim and recycle refrigerant and make repairs to keep you cool while driving.

Up next, vehicle repairs and maintenance tasks don’t always have to be done in the shop. You can easily do these 105 super-simple car repairs in your own garage.

All prices and links were current as of publication.

Robert Lacivita
Bob Lacivita is an award-winning auto technician and career and technical educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics, as well as writing state, federal and organizational foundation grants, and helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into technical/vocational training, for more than 20 years. His work has been featured in Family Handyman, a Reader's Digest book and Classic Bike Rider magazine, among others. Bob and his wife lived through 20 years' worth of DIY home remodeling while parenting two (now grown) boys and now relax by watching their three fabulous granddaughters.