Car A/C and heater faults explained for Dallas-Fort Worth drivers

Two refrigerants whose service ports will not fit each other

EPA's motor vehicle air conditioning pages say HFC-134a has been the most common refrigerant since 1994, and that by the 1995 model year every new US vehicle with A/C used it. Automakers began moving to HFO-1234yf in 2012, and EPA says it is now used in the majority of light-duty vehicles. Toyota's 2021 Corolla owner's manual (as reproduced on carclubsusa.com), for one, says a label about the A/C refrigerant is attached to the hood; look for the under-hood A/C label on your own car rather than guessing from the model year.

EPA requires a unique set of fittings for each approved refrigerant, on the vehicle, recovery equipment, large containers and small-can taps, and says an adapter may not be used to convert a fitting. On the car, EPA's table specifies R-1234yf quick-connect ports at 17 mm outside diameter on the high side and 14 mm on the low side. Small cans of R-134a use a right-hand Acme thread; R-1234yf cans use a left-hand thread.

R-1234yf is rated mildly flammable (ASHRAE A2L). EPA's use conditions require systems that use it to follow SAE J639, including a warning label, a high-pressure compressor cutoff switch and pressure relief devices.

EPA Section 609: who may remove refrigerant, and where it goes

On its page for servicing motor vehicle air conditioners, EPA says refrigerant cannot be intentionally released to the environment. It also says anyone who services an A/C system for payment of any kind, including bartering, must be certified under Section 609 of the Clean Air Act and must use refrigerant handling equipment certified by EPA or by an independent testing organization EPA approves.

Recovered refrigerant has to be recycled or reclaimed before it is recharged, even into the car it came out of. EPA explains that recycling removes impurities and oil, while reclamation returns refrigerant to virgin specifications. This is not legal advice. To see how the rules apply to your situation, check EPA's page or contact EPA.

Top off, evacuate and recharge, or repair the leak

AAA describes the cycle this way: the compressor, usually driven by the engine, pressurizes refrigerant gas and sends it to the condenser, where it releases heat and becomes a high-pressure liquid. The liquid then passes over evaporator coils in the dashboard, cooling the cabin air. AAA lists warm air as a sign that can mean low refrigerant or a malfunctioning compressor. AAA's outline is a summary and does not list every part.

EPA's page on recharging names three routes: topping off, evacuating and recharging, or repairing leaking components and then recharging. Topping off or recharging should improve cooling, EPA says, but neither service permanently fixes a leak; repairing or replacing the leaking components, if done properly, lasts longer. EPA notes that technicians cannot tell how much refrigerant is in a system when it arrives, so a top-off involves guessing, and an undercharge or overcharge both impair performance.

EPA says technicians can find leaks most of the time, especially with an electronic detector certified to SAE J1627, though small leaks can be hard to locate. A few ounces, about 10 percent of a normal charge, is enough for a leak check, and EPA adds that in some cases refrigerant must be added to find the leak. EPA does not require leak repair before charging, and notes that some states or localities may.

A compressor that will not engage: clutch, coil, lockout or fuse

A compressor remanufacturer says most manufacturers use an electromagnetic clutch. With the engine running and the A/C on max, it says to look for the clutch and pulley turning together, and that only the pulley turning indicates a problem somewhere in the system. Keep hands, hair and loose clothing well clear of the belt and pulley if you look. The remanufacturer lists a low-pressure lockout, a poor ground, a bad clutch coil, an open wire to the coil or a blown fuse, and suggests checking the clutch fuse first.

A technical article from the Mobile Air Climate Systems Association (MACS) says, in a piece written for technicians bench-testing a compressor before installing it, that a clutch with no function at all is usually due to an open clutch coil, and a coil whose resistance is too low is the most common cause of blown clutch-circuit fuses. Published specifications for 12-volt coils do not agree; the ranges the article found ran from 2.0 to 5.0 ohms at the widest, and resistance rises when the coil is hot. It says to use the vehicle-specific figure when one is available.

Warm, weak, musty or foggy: blend doors, filters and the heater side

PartCatalog's guide to blend door actuators describes the part as a small electric motor, usually near the heater core or evaporator, that moves the door controlling how much heater-core air mixes with cooled air. It lists a burned-out motor, stripped gears and a broken arm among the failures, and says a failed actuator can leave only hot air, only cold air or no way to adjust.

AAA says a clogged cabin air filter can bring weaker air pressure, foggy windows that take longer to clear and musty smells. The filter is often behind the glove box or under the dashboard; AAA says reaching it there may mean removing the fasteners holding the compartment or trim, and suggests a mechanic in that case. It puts the usual replacement at every 12,000 to 15,000 miles or once a year, and adds that the owner's manual lists the recommended interval.

The heater runs on a different system. The AA, a UK motoring organization, says most heaters pass engine coolant through a heater core with air blown across it, so air feels cold for the first few minutes until the engine warms the coolant; if it stays cold, it names low coolant and a thermostat fault as the usual suspects. One cross-over to know: Toyota's 2021 Corolla manual says switching the A/C on dehumidifies the air and defogs the windshield effectively.