MAXTEMP Error with hotend/nozzle

Hello -

I have recently finished building a Prusa i2 that I started 4-5 years ago. It has been working well for about a month but recently I have been getting MAXTEMP errors when printing. The errors appear to be associated with the nozzle - I'm printing in ABS at 235C and when I look at the temperature graph in Pronterface it displays 299!

I'm confused why this would happen because up until recently I was printing without errors, and now I get these huge temperature swings. I want to catch it before something gets damaged, but I'm not sure what would cause this and why.

Any suggestions? Thanks.

Is it really 299? Or really still 235 (or less), but the temperature is reading wrong

Broken thermistor wire? Or near-broken?

I remember when I was working with a K8200 printer, when the thermister shorted out on the heat block the temperature displayed was 750°C (obviously it wasn't!!!)

connect the printer to the computer and watch the temperature graph, with out heating the nozzle jiggle around the thermister wire near the heatblock and see if you get any temperature spikes. If you do it would indicate a short.