Thermistor reading incorrect temperature

Hi. I have recently build a printer with a 100k thermistor. I used marlin and configured the thermistor to its settings(it was a no brand thermistor) and the printer worked for a few weeks. However, a few days ago the printer suddenly started saying it was 246 c inroom temperature. I touched it and the extruder hotend was cold. After thinking this was a thermistor break I changed into a new thermistor(the same type). But it kept on reading about the same incorrect temperature. Can you help me on fixing this? I will provide more information if needed. Please help me out.

I also checked the resistance of the new tjermistor, and it starts at 105 k but slowly drops to 90k.

With the fact that you changed the thermistor and still get the high reading, unless your second thermistor came from the same lot as the first, your thermistor is probably not your problem. If you got them from the same source, try a different source. Your problem is probably your sensor shield. In the case you are using a Mega 2560, your sensor shield is included in your attached RAMPS 1.4 shield. In cases of integrated boards such as the GT2560, Sanguinololu,Rumba, MightyBoard, Megatronics V2.0 board and many other boards, the RAMPS 1.4 functions are already integrated into the board. If you are using a Mega 2560 board with a RAMPS 1.4 Shield just replace the shield. You can get generic versions of RAMPS 1.4 Shields for $11 with two day Prime shipping or $7.88 with free shipping from Amazon, depending on how fast you want your item and whether you bought into Prime. You can probably get one even cheaper on e-Bay but wait time may be long. You may even want a name brand such a SainSmart or Geeetech, but they will cost more. If you are using any of the integrated boards you will need to replace the whole board. Prices will vary.

Since you are getting a high reading, I would expect that no power is going your ceramic heater as you are seeing.

Edited 3 time(s). Last edit at 03/03/2016 04:18PM by Lymphomaniac1.

I actually used the same thermistor thinking it was a individual thermistor failure.
I am using NTC 3950 thermistors. Is there a way if i can check the shield is really malfunctioning?

The best way I have found to troubleshoot individual parts is to keep spare parts on hand and replace suspected parts with the spares. Most parts are relatively cheap. This keeps me from banging my head against the wall while troubleshooting and I always have replacement parts on hand so you do not have to wait for parts to be shipped from China.

Are you measuring the thermistor in its circuit? That won't give the right results, and I would expect a thermistor alone to be more stable than that at room temperature.

Are your thermistor settings stored in the firmware or EEPROM? Is there any way they got inadvertently reset to some defaults that are inappropriate?