thermistors wrong temp reads

Hi, i've a v1.2 gen7 board running fine except thermistor.
I have a thermistor from parcan and others from GRRF, all are 100k.
My RT1 and RT2 resistors are 4.7K (yellow, violet, red and gold colors).
When heater is off, reads 3ºC instead 25-28ºC ambient temp.
When heater is active, several 3ºC reads (about 20sec reading 3ºC) then, temp go up.
Also, seen header is running in lowtemp due i must use about 240ºC forn a "correct" extrusion with PLA and more for ABS.
If i plug thermistor in bed instead extruder , same occurs with both kind thermistors (parcan and grrf).
Does anyone know what happend?
TIA.

My temptable:

// /usr/local/bin/createTemperatureLookup.py --r0=100000 --t0=25 --r1=0 --r2=4700 --beta=4267 --max-adc=1023
// r0: 100000
// t0: 25
// r1: 0
// r2: 4700
// beta: 4267
// max adc: 1023
#define NUMTEMPS 20
short temptable[NUMTEMPS][2] = {
   {1, 713},
   {54, 236},
   {107, 195},
   {160, 172},
   {213, 157},
   {266, 144},
   {319, 134},
   {372, 125},
   {425, 117},
   {478, 110},
   {531, 103},
   {584, 96},
   {637, 89},
   {690, 83},
   {743, 75},
   {796, 68},
   {849, 59},
   {902, 48},
   {955, 34},
   {1008, 3}
};

Hi Danx3!

I have a similar situation here. I'm using Gen7 v1.3 with Teacup und the Thermistors from GRRF.
When heater is off both Thermistor tell me something between 6 and 15 °C (at ~25°C) .
But when i start heating and heat rises over 45°C or so i get quite correct temperature results from my Board.(I've confirmed this with an external thermometer).

I hope you know that you have to user different temptables for different Thermistors?! The table you posted seems not to be right one for the GRRF 100k Thermistors.

For them the table should look something like this.

// r0: 100000
// t0: 25
// r1: 0
// r2: 4700
// beta: 3974
// max adc: 1023
#define NUMTEMPS 20
short temptable[NUMTEMPS][2] = {
{1, 916},
{54, 265},
{107, 216},
{160, 189},
{213, 171},
{266, 157},
{319, 146},
{372, 136},
{425, 127},
{478, 118},
{531, 110},
{584, 103},
{637, 95},
{690, 88},
{743, 80},
{796, 71},
{849, 62},
{902, 50},
{955, 34},
{1008, 2}
};

You could also try to calculate a new table with more entrys to get more accurate measurements.

Thanks, I was try that temptable also with wrong result too.
I think the ADC reads >1024 and get below 1024 when more 50ºC or so...
i'll try to recheck the 4.7k resistors with a tester or change with other fresh...
I'm using now 254ºC for ABS an seen need a bit high temperature. I'm syspect 254ºC reads is really about 200ºC but haven't way to test with thermometer. No flames, no yellow lines...
I was try to make some temptables with the phyton script and concluded is a resistors problem...
Thanks again.