DIAGNOSIS: The infinite switch (the control knob's internal component) for that burner is failing or the burner element itself is wearing out. The switch is the more common failure.
FIX:
- Unplug the range.
- Lift the cooktop (it's heavy—prop it with a wooden block). You'll see screws at the front corners and sometimes along the sides.
- Locate the infinite switch connected to the problematic burner's knob. It's a square component with wires and a shaft the knob fits on.
- Disconnect the wires (note positions or take a photo), remove the mounting screws/nut, and replace the switch.
- Reconnect wires, lower cooktop, and test.
IF THAT DIDN'T WORK: Swap the suspect large burner element with a known-good small one from another position. If the problem moves, replace the burner element. If it stays at the same location, the wiring harness or socket could be faulty.
HEADS UP: When lifting the cooktop, be extremely careful of the wires connecting the controls and elements. Don't pull or strain them.