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Many of these appliances provide motor speed control, sometimes with a fully variable speed or several speed selections. Usually, a universal motor speed control will use a diac-triac arrangement to control motor speed. Either the triac or the diac can fail, but it is often the diac which has failed and the common sign of that is that the motor will run at full speed, but not at any reduced speed. See How dimmers and motor speed controllers work for their theory of operation and how to fix them.