By replacing the heat controls, inspecting the electrical spade connectors for spring tension and corrosion, measuring voltage at both controls while operating to rule out low voltage at the application and advising client to use only a cloth barely moistened with dish soap for cleaning in the vicinity of the switches on the range top. My personal feeling is that the appliance was (mistakenly??) equipped with the incorrect (intended for 6 in elements) switches at the factory, as the two I replaced were the same as what the range was equipped with for the 6 in elements.