When certain failures occur with the triacs driving the printhead's jetstack heaters, the printer declares a 37,002.47 or 37, 003.48 PEST fault. If the triac is failing in a constantly conducting state, the printhead jetstack heater will progressively get hotter. The printhead features two thermal cutouts, one for each jetstack heater. Normally the cutout will open and prevent a thermal runaway.
If the printer's power is continuously cycled it is possible for the printer to pass the PEST and continue the warming up cycle. However, the firmware may not have control of the printhead's temperature due to a short-circuit jetstack triac. Some control of the jetstack heater can be provided by the thermal cutout in the printhead, but eventually this may fail and thermal runaway occur.