Take its temperature. A healthy air-conditioning system should produce cold air within a few minutes. Turn it on with the temperature set to full cold and the blower at medium speed. Then keep it running when you road-test the vehicle. Be wary if the air coming through the dash vents turns warm and stays that way. While the problem could be minor - a faulty switch or excess moisture in the system, for example - a shift cold air to warm could mean an expensive repair bill down the road.
Know what's in there. A decal on the underside of the hood should reveal which refrigerant the factory installed or whether the vehicle was retrofitted with something else.
Unfortunately, decals can't tell you whether the original system was properly maintained or how well any retrofits were performed. That's why the surest way to know which air-conditioning system a vehicle has and what shape it's in is to have it checked by an air-conditioning specialist. An air-conditioning shop can use an electronic leak detector and trace dyes, if needed, to find any leaks. The shop can also inspect the system to see if it contains more than one refrigerant. Refrigerant mixes pose added problems and expense because purging them requires special equipment. And while both R-12 and R-134a can be recycled, blends must be collected and shipped off-site for reclamation - another expensive procedure to go through. Few shops are equipped to service systems with contaminated refrigerant, which alone makes the vehicle worth less. Mixes are also a telltale sign that the system was leaking and probably wasn't fixed before the other refrigerant was added. Worse, if the system has been filled with propane or some other flammable gas and it leaks into the passenger compartment, the gas may cause a fire or an explosion.
If the compressor needs replacing, the new one that goes in will probably be R-134a-compatible anyway, so it makes sense to switch to the new refrigerant. A specialist can tell you for sure and give you an estimate that you can use as a bargaining chip to lower a used vehicle's price. Then again, if the cost to repair or convert represents a significant portion of that price - and the owner won't discount it accordingly - you may want to pass on the vehicle and continue your search.
LOOK IN THE TRUNK
The trunk is another place to use your nose as well as your eyes. Again, sniff an look for signs of water entry. See if the carpeting feels wet or smells musty. Take up the trunk floor and check the spare-tire well for water or rust.
Check the condition of the spare tire. (If the car has alloy wheels, the spare-tire rim is often plain steel.) With many minivans, pickups, and sport-utility vehicles, the sp tire may be suspended beneath the rear of the vehicle. You'll have to get down on your knees to examine it. Also make sure the jack and all the jack tools are present and accounted for.
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