I have the following problem,

The heater of the 3D printer 12v- 40w gets hot only after warming the thermistor manually above 5 degrees. After we heat it up manually there is no problem and we can print. However when we don't manually heat up the Thermistor then it will not heat up at all. The electronics and the 100 NTC G540 (thermistor) and extruder work like a charm after the manually heating so they are fine. We already have replaced the RAMPS 1.4 and the Thermistors with new ones.
The temperature in Pronterface and on the LCD screen are the same as we measure with an IR temperature meter.
When we don't manually heat then in Pronterface we can't activate the temperature and it shows T 0/0.

When I change the settings in Marlin to #define HEATER_0_MINTEMP 0 it still doesn't heat up. We have tried 4 different Thermistors and they all gave the same result.

When we use these settings nothing works unless we pre-heat manually.
#define HEATER_0_MINTEMP 5
#define HEATER_0_MINTEMP 5
#define HEATER_0_MINTEMP 5
#define BED_MINTEMP 5I hope someone here has some suggestions on how to fix this problem.