Burner only cooks on high heat.

You need the switch for that burner. The contacts inside are stuck together. You will need to pull the unit and separate the glass from the lower body to access it. To get the right switch you need full model and serial number located on a plate at the bottom of the unit.

Your ability to perform this is dependent on how mechanical you are. The Whole unit must be removed from the counter opening and the base unscrewed from the glass, remove knobs before disassembling. For someone mechanical this is not difficult. De-energize whole unit first.

Once the two pieces are separated the remainder is mechanically easier. Disassemble the box on right containing the switches and replace the correct one. Reverse the process for reassembly. Good Luck