F21 error on induction cooktop; affected zone will not heat.

Safety first: Induction modules contain high-voltage capacitors. Disconnect power at the breaker and wait several minutes before opening the unit. This repair is advanced.

• Confirm zone: Note which cooking zone shows F21. Test other zones to see if they operate normally.
• Access internals: Remove the cooktop from the counter and take off the bottom cover to expose the coils and power modules.
• Inspect coil and wiring: Check the affected zone's coil for cracks, burns, or detached mounting. Ensure the coil connector is firmly seated on the power module.
• Test coil continuity: With a multimeter, measure resistance of the coil; an open circuit indicates a failed coil that must be replaced.
• Replace power module: If the coil is good but F21 persists, the associated power module is likely defective and should be replaced as a complete assembly.

Due to the complexity and risk, many users should have a qualified technician perform this repair.