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Thermistor problems permanently 49 °C and 74 °C

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I have recently purchased a Creality CR-10 MAX used.

After fixing some problems, the last big problem is a measurement error of the temperature sensors for heat bed and nozzle. The nozzle temperature is permanently displayed with 49 °C (at room temperature and during heating) and the heat bed temperature with 74 °C (also at room temperature and during heating). I have already replaced both sensors which has not led to any change. Subsequently, I have reloaded the firmware, which has also led to no change.

Now I am running out of ideas. As a last possibility I see only the exchange of the mainboard.

Do you have any ideas what the problem could be?

Thank you very much and many greetings,
Sebastian

A bit late, but here are my ideas.
I guess you have checked that the thermistors are properly connected to the mainboard. Getting fixed values for both sensors after a proper firmware flash and no wiring problems indicates usually that the A/D-port of the controller (usually AtMega2560) is not working any more. Usually the internal Aref logic is destroyed and your machine needs a new board. However it is worth to substantiate this devastating thesis first:

  1. A first check would be to unplug the sensors one after the other and see what happens. Any changes in the printers display (0/0°C) would indicate that there is life and it is worth to head on with steps 2 and 3. Otherwise jump to step 4.

  2. Connect a 100k potentiometer instead of a sensor to the board and (moderately) decrease the resistance starting from 100K. This simulates an increase in temperature. Have look at https://www.yumpu.com/en/document/view/41176608/100k-thermistor-output-table-100k-thermistor-output-table-bapi to see how the resistances correspond to the temperatures. If this is reflected in the display your board is fine.

  3. Use an Ohmmeter to check which resistances the sensors have and warm them up a bit, to see how the act. Again refer to the table. If they are alive go to step 4, otherwise find working ones.

  4. At this point it's worth to try another mainboard. However, if you are a programmer you can e.g. use the Arduino IDE and write/upload a simple sketch that lets you monitor the values of the I/O ports used for A/D via USB and upload it to the board. If the values change in response to changes applied via potentiometer (step 2) or sensor the mainboard should be fine and the problem points to the firmware.

I guess your path will be: step 1, jump to step 4. New board. Yeaaahh.

Have fun, anyway