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If the burner only works on high, it means that the burner switch itself under the cooktop will need to be ordered and replaced. This is the part that your knob itself attaches to which you turn to adjust the temperature of the burner under the cooktop. You'll need to uninstall the entire cooktop and remove all the screws under the glass top around the border to pull the glass top up and off so you can replace the part if you're handy enough to do that. There really isn't a reason to replace the cooktop for this issue, but it will probably cost about $250 to have a technician come and replace the part with part, trip, and labor if you have someone repair it for you.
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I can't tell which switch you need because the breakdown diagram is terrible. You'll need either part 7, 7A, 7B, or 7C on the diagram in the link below. You will need to call them directly to see which one you need however.
CLICK HERE for switch