Here's a statistic that should give contractors and technicians pause before they condemn a compressor at a jobsite: One-third of all compressors pulled from the field, returned to manufacturers, and torn down for failure analyses do not have observable defects. In other words, these compressors were most likely good, workable compressors when they were replaced.

Such a phenomenon has been taking place for years, and it even has a name. It's called the "no defect found" (NDF) compressor return.