OKIDATA ML320 Turbo Service Manual page 30

A thermistor in the printhead prevents burning caused by overheating of the coil during extended
continuous bi-directional printing. When the temperature of the coil exceeds a predetermined limit (about
119°C) the control circuit detects a thermistor signal. Printing will then be intermittent or stop completely
until the coil temperature falls below the limit value.
Figure 2-3