Most automotive A/C compressors will give clear indicators of imminent failure or problems with lubricant levels through a series of sounds and mechanical issues. Determining the cause of the A/C compressor issue, and determining if the compressor requires replacement, will take the average backyard mechanic about 30 minutes.
Determine the issue with the compressor by how it behaves when switched on. Start the car and set the A/C to maximum, listening for noises from the compressor. A bad whine or ratcheting sound could indicate low lubricant. A high, loud squeal could be caused by a locked or seized compressor or clutch, and the car should be switched off immediately to avoid belt damage. If the clutch does not spin, or is seized, the compressor will need to be removed (at a minimum) to be rebuilt.
Charge the system with the can of lubricant to quiet a noisy compressor. Tap the can of lubricant by screwing it onto the base of the freon recharge valve. The valve will have a needle on the inside of the base, puncturing the can and pressurizing the valve. Attach the valve's nozzle to the low pressure side of the A/C system. This can differ by manufacturer, but it is typically located on or near the evaporator canister. Start the car and turn on the A/C. Open the valve and discharge the lubricant into the system. The compressor noise should go away quickly. Once the can is emptied, usually in about five minutes, unscrew the can from the valve and repeat the procedure if more lubricant is required.
Check for leaks at or near the compressor and its fittings by adding a can of UV dye to the system in the same manner as the lubricant. One can is usually enough. Once the dye is in, run the system for a day and then check the entire A/C system for leaks with a UV black light. The dye will glow intense green to indicate a leak, and if the leak is on the compressor casing or from the internals, the compressor must be replaced.
