Samsung · Microwave Serious

C-21 — Cavity Over-Temperature

The thermistor inside the cavity exceeded the safe upper limit. The control aborts the cycle to protect the magnetron and cavity.

Keep using it?
Limited use only
DIY difficulty
Hard
Verified against
samsung.com ↗

How to fix it

  1. Stop using the microwave — let it cool fully (30+ minutes) before further testing.
  2. Never run a microwave empty; place a microwave-safe cup of water if testing.
  3. Clean or replace grease/charcoal filters; clear all vents (top and bottom).
  4. Confirm the cooling fan spins on power up; check for blockage or seized motor.
  5. Measure the cavity thermistor — typically ~50 kΩ at room temperature; replace if out of spec.
  6. If cooling and thermistor are good, suspect the magnetron — service call recommended due to high-voltage hazards.

Why it happens

Typical signs: C-21 mid-cycle · Hot smell from vents · Cycle aborts and cooling fan keeps running

  • Running empty or near-empty No load to absorb microwave energy causes runaway cavity heating.
  • Blocked vents / clogged grease filters (OTR) Restricted airflow cooks the cavity.
  • Failed cooling fan or stirrer fan Magnetron and cavity overheat without normal airflow.
  • Failed cavity thermistor Reads high even when temperature is normal.
  • Failing magnetron Inefficient magnetron dumps heat into the cavity.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

  • Samsung microwave cavity thermistor Common replacement
    Amazon →
  • Microwave grease/charcoal filter Common replacement
    Amazon →
  • Microwave cooling fan motor Common replacement
    Amazon →

More Samsung microwave codes

5E 5E / SE — Touchpad Shorted (Older Spelling) C-22 C-22 — Temperature Sensor Error C-A0 C-A0 — Functional Error C-A1 C-A1 — Functional Error (Damper / Sensing) C-A2 C-A2 — Functional Error C-d0 C-d0 — Door or Key Error

All Samsung codes → All microwave codes →

Frequently asked questions

What does the C-21 code mean on a Samsung microwave?

The thermistor inside the cavity exceeded the safe upper limit. The control aborts the cycle to protect the magnetron and cavity.

What causes the C-21 code on a Samsung microwave?

Running empty or near-empty: No load to absorb microwave energy causes runaway cavity heating. Blocked vents / clogged grease filters (OTR): Restricted airflow cooks the cavity. Failed cooling fan or stirrer fan: Magnetron and cavity overheat without normal airflow.

Is it safe to keep using a Samsung microwave with a C-21 error?

Limited use only. This is rated a serious-severity fault.

How do I fix C-21 on a Samsung microwave?

Stop using the microwave — let it cool fully (30+ minutes) before further testing. Never run a microwave empty; place a microwave-safe cup of water if testing. Clean or replace grease/charcoal filters; clear all vents (top and bottom).

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: samsung.com.