The short answer
The short reading of F21 on a KitchenAid cooktop: there is a problem with the power supply frequency. Stop using it until the supply has been checked: an induction cooktop depends on a stable supply to switch its zones. Most of what follows is diagnosis rather than repair, and the repair end of it is a technician job.
Error codes
Reference| Code | Meaning |
|---|---|
| F21 | There is a problem with power supply frequency. |
What the machine does alongside the code
- Zones cut out or refuse to start with no pattern to the cooking — A supply fault follows the building rather than the pan.
- The code appears at the moment a heavy load switches on — Heating and motor starts are when a marginal supply shows itself.
What triggers F21, most likely first
- A supply that is not a stable mains source — It goes first because it is the explanation that actually produces a frequency error: a generator, an inverter or a temporary supply does not hold frequency the way a mains connection does.
- A loose or corroded connection at the outlet or the terminal block — A joint that is merely tight enough passes a small load and collapses under a large one, which is exactly the pattern the code records.
- A control board that misreads the supply it is given — It is last because it is the least likely, and the only way to reach it honestly is to prove the supply itself is sound first.
Check in this order, and why
- Establish what the property is actually supplied from — Mains, generator or inverter: that single answer explains most frequency readings.
- Have the supply measured under load, not at rest — A supply reads fine with nothing drawing from it; the measurement that matters is taken while the appliance is drawing its heaviest load.
- Ask whether anything else in the building behaves oddly at the same times — A shared symptom moves the investigation to the supply.
- Body: Separate the appliance from the building first: one run with the rest of the circuit switched off answers more than any measurement.
The reset, then the owner-safe checks
- Switch the cooktop off at the breaker and establish what supplies the property before restoring it — If the answer is anything other than a stable mains connection, that is the finding.
About 15 min
- Run the appliance with the rest of the circuit idle — Switch off the other heavy loads on the same breaker and repeat whatever produced the code.
About 45 min
- Reset the breaker once and watch it — Cut the breaker for a minute, restore it, and note whether the code comes back immediately or only under load.
About 5 min
- STOP HERE — supply measurement is licensed work — Do not open an outlet or a terminal block to check a voltage. That is where an owner check ends.
Before anything is opened
Safety
Switch the cooktop off at its circuit breaker before anything is opened; it carries a heavy load and stored charge remains after the power is off. Let the glass cool on its own. On a gas model, never switch anything on or off if you smell gas: leave and call the gas utility from outside.
When this is a service call rather than a reset
Call an electrician first if a faceplate is warm or discoloured, if the breaker trips, or if the code tracks the time of day rather than the programme.
Neighbouring codes on KitchenAid cooktops
What this page does not cover
This reading applies to the KitchenAid cooktop line as a whole, not to one family or model. It does not cover what differs between model lines — wiring colours, connector positions, part numbers and service-mode sequences — which are in your own model manual and which a technician matches to the rating plate rather than to this code. Where a family- or model-specific page for this code exists, it is linked from the code index below and it is the more specific answer.
Links: - [KitchenAid cooktop error codes](/blog/kitchenaid-cooktop-error-codes)
Where to go next
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
