Ricoh K-C3.5L Service Training page 109

Three levels of overheat protection
Normally, the primary level can fully protect the hardware. The
second works as the fail-safe feature for the first one. The third
works as the fail-safe feature for the second one.
Primary Level:
If the fusing temperature reaches 230°C (or higher) and stays
so for one second, the controller turns the fusing lamp off. In a
case like this, SC543 or SC553 shows.
Second Level:
If the fusing feature reaches 250°C, the controller cuts off the
24V line. (The fusing lamps are on the 24V line.)
Third Level:
Two thermostats are attached on each line of the two fusing
lamps. (four thermostats in total). One of the two thermostats
cuts the power supply to the fusing lamp at 179°C, and the
other cuts the power supply at 180°C. (Note that the thermostat
temperature is somewhat lower than the fusing temperature.)
Slide 109
More on Overheat Protection
There are three error codes associated with thermostat failure.
SC544 is related to low temperature detection
SC546 is related to unstable fusing unit temperature.
SC541 occurs after resetting the above codes, if the thermostat is still open.
Overheat Protection
109
Model K-C3.5L