Sometimes the thermostat shows LL or HH, HH means the temp has exceeded highest point and LL means the temp has exceeded lowest point.
Try again a few minutes later.
But most likely, HH means sensor short circuited; And LL means circuit broke. This happens when the warmer has been wrongly used, so the alignment of the sensor is no long in correct position. It could be over folded, corrugated or wet.
Sometimes it is because of high frequency distraction from other devices;
For such incident we provide 1x replacement.