Troubleshooter: What to do when A/C blows hot air

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Pretty much every new vehicle now comes with air conditioning, right down to the Mitsubishi Mirage. It’s a good thing, too: whether you put it down to global warming or entitlement, we all seem to love our cool air on a humid summer’s drive. Unfortunately cars don’t stay new forever, and it’s common for air-conditioning problems to crop up as most rides hit their fourth or fifth birthday. If you’re like most drivers, you’ll recognize pretty quickly when its dash vents start pumping hot, sticky breezes.

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As not every shop does air conditioning repairs, many consumers head back to their dealerships when their vehicle’s air vents blow more hot air than a politician. But as there are usually large dollars involved in even a simple repair, doing a little legwork and asking the right questions can pay off big time, while still leaving you as cool as the proverbial cucumber.

Why isn’t my air conditioning blowing cold air?

If you’re new to driving a vehicle with A/C, remember that before you feel any cooling comfort after starting the car on a hot day, you’ll have to get all the hot air out. Pop one rear window slightly as you drive with the A/C on to quickly evacuate all of that steamy air. A sunroof with a rear pop-up feature works even better.

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If the system is indeed down, the likely cause is a leak of refrigerant gas. Vehicle A/C systems are not much different that the window-shakers in your apartment or the sleep-robbing ground-mounted units used for central air: they contain a compressor to pressurize and circulate the refrigerant gas, a heat-exchanging condenser, an evaporator coil for the cooled gas to flow through, and a fan to move the air through the evaporator and onto you. The compressor is mounted to the engine’s front and driven by a belt. The condenser (which looks like a miniature radiator) is mounted just behind the grille at the very front, and the evaporator is buried with the fan deep behind the dash.

What does a mechanic look for when air conditioning blows hot?

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The most common source of leaks is the condenser, which is mounted in the direct line of fire of any road-debris projectiles thrown up by traffic in front. Next are the cursory lines and hoses that connect everything together and provide pathways for the gas. It’s worth noting that apart from those at O-rings and joints themselves, very few leaks can be repaired themselves; most require replacement of an entire leaking component.

A good technician will start by checking the refrigerant gas level. If that pressure appears correct (meaning no leaks), the next process will involve verifying and repairing the control systems.

If the system is low on gas, then it’s on to a hunt for leaks. If the automaker was thoughtful enough to add a little leak-detection dye to the system from the factory (as many do these days), a black light can be used to spot the leak. With that identified, it’s time to prepare a repair estimate and locate the smelling salts for the customer.

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Dealers usually stick with factory parts when repairing A/C

For the most part, dealership service departments source repair parts and components exclusively from their factory warehouse systems and are hesitant to seek other solutions, but you’ll never know unless you ask. As an example, a common problem in the current Dodge Grand Caravan is a leaking rear evaporator. Like most minivans with rear A/C, the rear unit is equipped with its own heater core, A/C evaporator, and blower fan. The main system up front provides hot coolant for the heater and cooled refrigerant gas for A/C to the rear via a long set of flexible aluminum tubes.

However, bumps and vibrations can take their toll on the evaporator by creating small cracks where the tubes meet the finned structure. A new rear evaporator from FCA retails for $646, while aftermarket replacements can be had for as little as $120. Yes, the Dodge unit comes with an expansion valve attached, but it’s not necessary just to replace a leaking evaporator. Any HVAC repair that involves accessing either the front or rear housing is labour intensive, and therefore expensive proposition.

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An easier and cheaper alternative is to simply cut the rear A/C evaporator out of the system by blocking the feed and return tubes. This only requires two small, low-priced metal caps and a few hours of a tech’s time. Yes, it leaves you without rear A/C, but the front unit should have improved performance thanks to the reduced volume of circulation without the rear system being hooked up. This repair can come in under $450 or so, depending on shop rates, while replacing the rear evaporator can easily top $1,200 or more.

How expensive are air conditioning repairs?

Few quality A/C repairs tally less than $1,000. Today’s ozone-friendly refrigerant gas (R1234YF) can easily top $200 per pound, and most vehicles require 1-2 pounds of the stuff. If you own a minivan or larger SUV with a rear A/C system, those costs will climb further still, but these cases may also leave for some alternatives.

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A true auxiliary rear A/C system will have long set of hoses to carrying the refrigerant gas to the rear unit. These lines are a common and expensive leak risk, as their typical routing under the vehicle exposes them to all the junk and debris that we drive over. A much cheaper alternative is to have your shop remove and cap these lines. The front A/C system will probably even work a little better, as it won’t be bleeding cooled gas to the rear unit.

Aftermarket A/C parts can be an option

Other savings can be obtained by sourcing non-factory parts, when available. If you’re driving a particularly popular ride, you’ll find a surprisingly large number of A/C components such as compressors, condensers, lines, evaporators, and valves from a variety of companies. Sometimes, the price differences are small and therefore outweighed by better warranties with OEM components, and with certain parts, the automaker is the only source, but you’ll never know unless you ask.

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Avoid DIY air conditioning repairs

Many backyard DIY techs will scoff at a professional shop’s A/C estimate with an inevitable, “I can do it cheaper.”

Unfortunately there are no true shortcuts on air-conditioning repairs. Many auto-part stores sell stop-leak kits which claim to seal small leaks and recharge the system to restore normal operation, but there are two big problems with these kits. First, their alternative refrigerant gas is actually a butane derivative, which is highly flammable. Second, their stop-leak chemicals can easily gum up the works on a professional’s recharging equipment. When the fix doesn’t work and you take the vehicle into the shop (without telling them about the stop-leak), you can therefore damage a piece of their equipment that’s often worth more than the car you brought in.

If you think your vehicle’s A/C repair was expensive, wait until you get the bill for fixing their recharging equipment.

Updated May 24, 2023.