Somewhere, at this moment, there’s a guy in an auto parts store reading the label on a one-pound can of R-134a, the kind with a hose on it that you can hook up to charge the A/C system.

If you see that guy, tell him to put that can back.

A/C service is not a do-it-yourself job. What seems like a quick fix really is an invitation to an expensive, time-consuming repair.

Parts stores typically carry three varieties of canned refrigerant: R-134a with oil for the compressor; R-134a with oil and sealant; and straight R-134a. Each of them can harm your system.

For example, the oil mixed with R-134a may not match the compressor's original PAG or ester oil specification. If the compressor lunges, the manufacturer will analyze the oil for its type and viscosity. If it detects an oil that's different from the original, chances are the manufacturer will void the warranty.

Worse is R-134a with sealant. Sealant is designed to solidify when it contacts outside air—a leak. The problem is, sealant can also plug up valves and tubes. It can also harm refrigerant recovery and recycling equipment.

Why not use a one-pound can of straight R-134a?

First, no one knows whether a pound of refrigerant is the amount the A/C system needs. Second, if the system is losing refrigerant charge, it should be checked out by a qualified A/C service technician.