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Hello.

Specs:
DIY Rostock Max v2
Simplify3d
Repetier 92.2

I just flipped back to ABS after a couple months of printing primarily in PLA, and I can no longer get to 220c. Just a few days ago I was getting double def for temps (cold snap...I think), and now there's this fun to figure out. Per the other related threads, this seems like a potential dead resistor. I've run PID autotune and I am getting the same results of hovering just under 220c when explicitly set to that temp.

I can replace a resistor, but would I be better off just taking the new hotend or the e3d for enhanced reliability?

I think if it's hovering just under it (IE, within ~5 degrees), it's likely just a bad PID tune. autotune isn't perfect. Now, if you find there's a wall that you can't get over no matter the set temperature, then it may be a dead resistor. I would increase P and give it a shot.