I've got 4 Herpstats. 1 Herpstat 2 redline, 1 Herpstat 2 normal, 1 Herpstat 1 redline and 1 herpstat 1 normal and mine have been fine for over 4 years now. If they fail, they fail in the off position and/or you can set the safeties.
As for the OP, did you make sure the Hi/Low temp levels were set to your settings? If you got this used, as you found out, you need to hit the master reset to reset everything to default. Then go through each menu and set everything to YOUR settings.
Setting it to 90 and then it not going over 78 sounds like the pervious owner had the IO set to max at 78 which would explain why it wouldn't heat over 90 until you raised the Hi temp level to 90 or what I do, is 10F over what I want for a max level.
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