Something has gone wrong with my thermistor or hot end and I can't figure out what. Without changing anything, my PLA was all of a sudden being liquefied instead of printing correctly. It clogged and ruined my hot end, so I had to replace that and the thermistor. After replacing everything and running some tests it is still happening. It appears as though the thermistor isn't reading the correct values, even though I made sure I am using the right temperature table. In pronterface, the readings I am getting show the hot end heating up much slower than my previous one, but I highly doubt that is the case since at around 160 C (or so it says) my PLA is basically liquid and drips out in droplets instead of oozing like normal.

Its a brand new hot end and thermistor, so does anyone know what my problem could be?? Thanks.

Oh, that does not sound right...
I will take a stab at it-
Well if you are sure that the thermistor is the correct type and that the software is correctly configured for it, then it sounds like a physical obstruction to heat flow from the heater block to the thermistor or an electrical short or high resistance in the leads to the thermistor.
Did you check that the thermistor is in direct contact with the metal and in the correct location?
Or maybe the thermistor leads are grounding out on a metal surface?
Is there some paste or adhesive that could be conductive and grounding the wires out?
Try checking the electrical joints for resistance at the thermistor leads.
Maybe resistance test the connections to the electronic board to the thermistor leads, or just remove them and reconnect.

I have checked the thermistor and it seems to have good contact and be in the right location. Its in exactly the same spot as before the hot end ruined itself. I have removed and re-attached the thermistor leads with no success.

Is there any easy way to test the thermistor and ensure that it is working correctly?

I'm no electrician but it just seems like getting an accurate reading from a thermistor shouldn't be that hard. When I first built my printer I literally just plugged it in and it worked perfectly so I can't understand why it is all of a sudden having such a problem.

I ran a few more tests and discovered that the heater was way overshooting during the heating process, but I'm not sure what that means.

I set my hot end to 100C and then it reached 115C before it finally began dropping again. Then I set it to 160C and it reached 177C before dropping back down to around 160C. At this point I had liquid PLA dripping from the hot end. No bueno. So I lowered the temp to 140C. When it got there i could push the filament through by hand and it came out in a steady ooze like it usually should, which means the hot end was not really at 140C. The temperature was also fluctuating wildly between 137C and 147C even though it should have been stable at 140C. At that point I gave up for the evening.

If anyone has other suggestions as to the problem, I would appreciate it. Hopefully some of this testing can help somebody narrow down what my issue is, because I am stumped at the moment.

Have you measured the hotend temp with a multitester lead? Or are you just going by the temp reading in your display/printer interface? What's your display/printer interface read for temp when hotend is off and room temp?

It reads about 21C when it is off and room temp. All of those values were what was given to me by pronterface. I tried figuring out how to measure/calculate the temp using a multitester, but I couldn't figure it out.

I've done a lot more testing and basically gotten nowhere with this problem. If anybody else has any ideas or help I would greatly appreciate it!!

Have you got a Ramps board? If so, check that the 10 ohm resistor (R13) is sound and properly soldered, and that there is continuity between Q1 (left hand pin as you look at it from the side) and the resistor. Also check that there is continuity between R13 and D10.

I checked and they both have continuity. After asking around on the IRC I was told to try setting the PID heater control in my firmware and that seemed to do the trick! It now heats up great. Unfortunately I think my nozzle is jammed now.... back to square one

How did you set up the PID?

In Sprinter and Marlin (I believe) there is an auto PID setup function that can be enabled in your firmware. Then run the M303 command in pronterface and the software will handle the rest. It will heat up the hot end and run through a few cycles. Then it will give you some numbers to put into your firmware. I did that and now it heats up fantastically and accurately.

Thanks Alex