The short answer
F21 is published as a problem with power supply frequency, and that wording points outward rather than inward. Mains frequency is set by the grid, so a cooktop complaining about it is almost always being fed by something that is not the grid — a generator, an inverter, or a backup supply. The cooktop is refusing the supply, not failing on it.
Whirlpool cooktop code as the manufacturer publishes it
Reference| Code | Meaning |
|---|---|
| F21 | There is a problem with power supply frequency. |
What you observe alongside the code
- The code with no zone working at all — A supply-side fault takes everything with it.
- Other large appliances misbehaving at the same time — Moves the fault upstream of the cooktop.
- It appears only on a generator or an inverter supply — The classic cause for a supply-quality code.
- Lights dimming when a zone is turned up — A voltage drop worth an electrician regardless of the code.
What triggers it, most likely first
- A supply that is not clean mains — First: a generator, an inverter or an undersized backup supply delivers power a cooktop control can reject outright.
- A utility-side problem — Last: rare, real, and the one case where nothing in the house is at fault.
- A loose supply or neutral connection — Third: a terminal that has backed off gives a sagging, noisy supply and heats where it is loose.
- Voltage drop on an undersized or long run — Fourth: it shows up under load, when the cooktop draws hardest.
Check in this order, and why
- Establish what is actually supplying the cooktop — Grid, generator, inverter or a backup system — it decides everything else.
- Check whether anything else in the house is affected — A yes puts the diagnosis outside the appliance immediately.
- Ask whether the cooktop is running on a generator or inverter — It explains most supply-quality codes on its own.
- Reset the breaker fully off and on — A half-tripped double-pole breaker feels on and is not.
Clearing the code, then the owner-safe checks
- Switch the cooktop breaker fully off, then fully on — Both poles; a partial trip is a common and invisible cause.
About 5 min
- Try the cooktop on normal mains if it has been on a generator — The cleanest test of a supply-quality fault.
About 15 min
- Note whether other appliances flicker at the same moment — Free evidence, and it decides who you call.
About 10 min
- STOP HERE — supply testing is an electrician’s work — Live terminals at cooking current are not an owner check.
Before you open anything
Safety
Switch the cooktop off at its breaker before any panel comes off — a capacitor can hold a charge after the power is off, and a cooktop is fed from a double-pole breaker, so both poles have to be dead. The glass stays hot long after the zones are off. Never bridge or bypass a thermal cut-out or an interlock to keep cooking. A loose supply terminal heats where it is loose, so a warm breaker or a smell at the panel means stop and call an electrician.
When this code is a service call
Call an electrician rather than an appliance technician: the suspects are the breaker, the supply terminals and the circuit feeding the cooktop, none of which is inside the appliance. Call an appliance technician only once a clean, correct supply has been measured at the cooktop and the code still stands.
Neighbouring Whirlpool cooktop codes
Which model lines this reading applies to
This reading of F21 applies to Whirlpool induction and radiant glass cooktops with the touch control that publishes this code, which is where the manufacturer documents it. A knob-controlled coil or gas cooktop has no display and cannot produce it, and a Whirlpool-built unit badged for another brand may pair the same characters with a different fault. Tolerance for supply frequency differs between induction generations, so one Whirlpool cooktop may accept a supply that another rejects. Where a model-specific page exists for your unit it is linked above and is the more specific answer: module part numbers, connector positions and resistance figures differ by family and belong there.
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
