Listen. Let the client talk and explain in detail exactly what they feel is wrong and what should be done. Let the client vent and remain quiet; fight the impulse to interrupt and give them an immediate solution. When a client is excited, belligerent, and/or acting in a discourteous manner, keep calm. If they're speaking louder, you should speak more softly. Clients frequently give wrong information because they don't really know how to technically explain the problem; in many cases, the complaint is rarely as bad as the client may have stated.
Investigate. Be sure and send a knowledgeable, qualified person to look at the problem, not just someone with technical know-how but someone having good people skills. Make sure you ask enough questions and ask the client to repeat anything that isn't clear; one can't begin to help until the extent of the problem is known from the client's point of view. Nothing is more frustrating to a client than a mistake in diagnosis or having to go through still another inspection.
Resolve quickly. After the problem has been investigated, and as quickly as possible, one must figure out best case and worse case settlement, i.e., the cost of the problem. Don't make the mistake of talking with the client to resolve the problem without knowing exactly how far you can go; you should know about how far if you asked the client about their expectations. If the client's idea of how the problem should be solved is either not possible, impractical, or beyond what is reasonable, it is best to quickly tell the client "No" with a smile on your face. Don't drag it out or try and avoid it. The final solution may not be exactly what the client would have preferred, however, if the problem is resolved quickly and in their favor one will be more likely to avoid significant frustration and law suits. The longer one has to wait for a problem to be resolved, the more the problem is magnified.