Definition
A detached sensor can report a low temperature while the heater continues adding energy, and a failed heater may not rise as expected. Firmware therefore checks initial heating progress, deviation from target, sensor plausibility and configured maximums using implementation-specific thresholds and models.
Marlin thermal protection, Klipper verify_heater and RepRapFirmware heater-fault monitoring use different settings and state machines. They reduce risk but depend on correct sensors, wiring, configuration and functioning electronics. Firmware vendors also recommend independent electrical and fire-safety measures.
What matters in practice
Detects implausible response
Protection looks for too little heating, excessive deviation, bad readings or configured over-temperature conditions.
Configuration matters
Thresholds that are too loose reduce protection; thresholds that are too strict can cause false faults.
Not provable from a file
Static G-code cannot test the sensor, heater, wiring, cutoff hardware or installed firmware settings.
Example in G-code
A target is not a safety test
; A G-code file cannot prove that heater protection works
M104 S210M104 requests 210 °C. The preceding comment correctly says the file cannot prove how real hardware responds.
What this viewer can show
The viewer can report explicit temperature targets but cannot verify thermal protection, sensor readings, heater power or shutdown behavior. A clean preview is never a heater-safety certificate.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.