C-21 Serious Hard — consider a tech
Samsung Microwave C-21 — Cavity Over-Temperature
Samsung Microwave · Error code C-21
What C-21 Means
The thermistor inside the cavity exceeded the safe upper limit. The control aborts the cycle to protect the magnetron and cavity.
Symptoms
- C-21 mid-cycle
- Hot smell from vents
- Cycle aborts and cooling fan keeps running
Common Causes
1
Running empty or near-empty
No load to absorb microwave energy causes runaway cavity heating.
2
Blocked vents / clogged grease filters (OTR)
Restricted airflow cooks the cavity.
3
Failed cooling fan or stirrer fan
Magnetron and cavity overheat without normal airflow.
4
Failed cavity thermistor
Reads high even when temperature is normal.
5
Failing magnetron
Inefficient magnetron dumps heat into the cavity.
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.
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
Verified against manufacturer documentation: samsung.com
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.