Months of printing, then suddenly MAX TEMP exceeded on hot end

I build a Mendelmax 1.5+ last year and have been printing successfully since then. Last week I printed a part then turned the machine off. When I came back to it a couple days later I was suprised to find that Pronterface was reading 750C on the hotend even without the printer on. I reset the printer but still got the same error. I haven't changed any settings for some time.

When I disconnect the thermistor from RAMPS I read 0C which makes me think there's an electrical short in the thermistor connection such that the wires are connected without the thermistor being in circuit. However, I'm not sure why this would have just suddenly manifested after so long. I do see a big blob of solid PLA sitting at the bottom of the hot end, which presumably oozed out after the last print, so perhaps that's somehow to blame.

Anybody experienced this, and if so what was the resolution? I don't want to start taking apart my hot end until I've excluded other possibilities, but so far I can't come up with any.

Thanks,
Mike

Max temp is caused by a short circuit. It could happen after a long time due to excess PLA carbonizing over the thermistor connection.

Thanks. If that's the case, how would I go about removing it?

I don't know how your thermistor is inserted. It might pull out or might need drilling out.

You can't just come up with a diagnosis without doing a little investigation. Disconnect your thermistor from your controller board and use an ohm meter to determine where the short circuit is. In my experience, issues like this almost always happen at connectors. I have had the heat wires fail on me twice and the thermistor once. Over time, that moving back and forth wears on the weakest link in the chain - and that is usually the connection point.