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
- 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).
- Confirm the cooling fan spins on power up; check for blockage or seized motor.
- Measure the cavity thermistor — typically ~50 kΩ at room temperature; replace if out of spec.
- 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.
More Samsung microwave codes
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.