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I have an e3d v6 and in the documentation, they warn you not to let the temperature climb over 297 C or else you'll damage the thermistor.

Well, last night things accidentally got way too hot and the temperature went to 320-330+. I don't know exactly how hot it got but I know it was hot enough to trip the firmware's valid range check and the LCD was showing "def" - the same as if you disconnected the thermistor entirely.

Today, the thermistor appears to still be reading valid values. I compared it with a thermocouple I have and it appears to be reading 4 degrees too hot, but since the spec sheet for the thermistor gives a tolerance of +/- 3%, that's probably okay.

My question is whether it needs replaced anyways. Are these things totally black and white where it's either 100% good vs. blown up, or is there a grey area in between where it's kind of okay but might cause trouble at some point?

Should be fine, when they fail it's pretty obvious.

Usually these gizmos have a range where they are accurate, a range where they are not so acurate , a range where they start getting damaged and a temp where they just pop.