Error code
C-21
Meaning
Oven internal overheating — oven temperature reaches 650 F (343 C) in any cooking function except self-clean
Moderate
What to do
Why this happens
Causes may include an oven temperature sensor malfunction, or relays for broil, bake, and DLB being stuck on. Additionally, a disconnected or damaged harness linked to the Main PCB could be responsible.
SourceSamsung service manual