"MAXTEMP triggered" but Extruder at 0 degrees.

CptAmmogeddon

Two Disclaimers:
A: The "Maxtemp"-issue isn't the only problem/symptom, just a part of the problem. I just thought it might be a relevant symptom for the title of the discussion.
B: This is an old and partially modified (not by me, but a friend who knows this kinda stuff but sadly doesn't remember what/how exactly he modified) Print Rite DIY 3D printer I am talking about. So I understand that it is probably pretty hard to pinpoint any problem.

TL;DR: Took out addon to Hotend-thermistor because of loose contacts/temp fluctuations. After that (and installing original thermistor):

Both heating elements can't be activated

Hotend-Temp is shown to be 0° consistently (Bed Temp seems to work)

Errors: "Temperature is reset. Set it before restarting" and "MAXTEMP triggered"

Homing works, controlling/moving any axis doesn't.

The road so far:
I had problems with fluctuating temperature readouts due to a loose contact [on an arduino-board that was added to the printer to be able to use a different Thermistor than the one that came with it (apparantly an NTC 100K)]. After trying to fix the contacts (as I had to do several times in the past), Repetier either said, that the Hot end was at 0°, or somewhere between 500 and 2000°. Since I didn't find the issue and the problem with fluctuating temps seemed to become more frequent anyways, I asked said friend if I could take that entire addon out and go "back to the roots" with a NTC 100K thermistor. Said friend told me that it shouldn't be an issue and the reason he added the board in the first place was because he couldn't find the original thermistor anywhere.
So I got to researching, dug up that the printer needs the aforementioned thermistor, purchased it (this one, to be precise: https://www.amazon.de/gp/product/B09HMCP5NW/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1) and took out the addon.
The Board was only connected to the Power Supply in parralel and obviously to the thermistor and it's output-port on the MB.
So in theory, I should have been able to just take out the board and connect a thermistor directly, instead.

The current problem:
Sadly, it now seems that Repetier doesn't recognize anything on my printer that has to do with temperature control:
Connecting works, it lists my Extruder and my heated bed (and reads out a seemingly normal temp for the bed at around 20°) and homing also works like a charm. But the Hotend-Temp is always at 0°, and as soon as I try to move an axis, it says "Printer stopped due to errors. Fix the error and use M999 to restart. (Temperature is reset. Set it before restarting)." and doesn't move, no matter which axis I try to control. It then also says "Extruder switched off. MAXTEMP triggered.", even though the temperature never changes from 0°.
Furthermore, I can't heat up the Hotend OR Bed. Repetier doesn't seem to notice anything wrong when clicking on the Extruder- or Bed-Icon to heat them up, but neither actually get hot, and the indicator LED on the heated bed stays off, as well. Obviously this could mean that the heating elements are broken, but it is a weird coincidence for both of them not to work, imo. Also, in my experience, Repetier doesn't even recognize the heated bed if there is a connectivity problem anywhere in it's circuit.

I also don't see any obviously broken or disconnected cables, and since everything worked until the aforementioned loose contact, and I think I was careful enough while disconnecting the Arduino-board, there shouldn't be a loose connection as far as I am aware.

Again, I get that it is hard to diagnose problems this way (especially since the printer is modified), but I still would appreciate any and all help I can get.

If it helps, here is the Log from Repetier after connecting the printer, homing it and trying to move an axis:
19:18:38.234 No start signal detected - forcing start
19:18:38.250 N1 M11034
19:18:38.250 N2 M11536
19:18:38.250 N3 M10536
19:18:38.250 N4 M11435
19:18:38.297 N5 M111 S698
19:18:38.297 N6 T060
19:18:38.297 N7 M2022
19:18:38.297 N8 M220 S100105
19:18:38.297 N9 M221 S100105
19:18:38.297 N10 M8042
19:18:38.297 FIRMWARE_NAME:Marlin V1; Sprinter/grbl mashup for gen 6 FIRMWARE_URL:http://www.mendel-parts.com PROTOCOL_Version:1.0 MACHINE_TYPE:Mendel EXTRUDER_COUNT:1
19:18:38.297 N11 M111 S687
19:18:38.359 X: 0.000Y:0.000Z:0.000E:0.000 Count X:0.000Y:0.000Z:0.000
19:18:38.359 echo:Active Extruder: 0
19:18:38.359 N12 T09
19:18:38.359 Begin file list
19:18:38.359 End file list

19:20:16.561 N13 G2833

19:20:21.342 N14 G1 X0 F480063
19:20:21.389 Printer stopped due to errors. Fix the error and use M999 to restart!. (Temperature is reset. Set it before restarting)

19:20:23.014 N15 M99930
19:20:23.014 N1 M11034
19:20:23.030 Resend: 1
19:20:23.061 Resend: N1 M110*34
19:20:23.061 Error:No Line Number with checksum, Last Line:0
19:20:23.108 Error:0
19:20:23.108 Extruder switched off. MAXTEMP triggered.

Repetier

You are using Marlin V1 firmware which is nor from us, so can not say much about firmware errors. Guess it at least reads high values causing it to trigge rthe MAXTEMP error.

You have the temperature filter active so in log it is not visible whch temperatures are measured from firmware. I think when you see it your self it might obviou sthat it comes from firmware. Means if it has same temperatures in M105 response it comes from the printer. This is in most cases a hardware defect especially if same settings worked in the past.

CptAmmogeddon

So, after activating the Temp readout and sensing M105, Repetier says:

ok T:-273.1 /273.1 B:18.7 /-273.1 @:0

Does that narrow it down?

Repetier

It say bed has 18°C but is disabled.

Extruder measures -273.1°C which I don't believe. So that seems like sensor cable not connected/broken/shorted or sensor defect. Due to all movements the cables might break at some time and no contact makes it measure extereme values.

What I do not understand is why temperature is set to 273.1 °C When you move our temperature slide (before getting maxtemp error warning) the value should change to a regular value you selected. Reading is

T:measuredTemp /SetTemp

CptAmmogeddon

So I played around with the sliders, but it still says -273.1/-273.1 and 18.7/-273.1 (sorry, I had a typo in my previous post, the set temp value is also negative).

Does that mean, it is a firmware/conmection issue? Since it doesn't even send the set temp back?

What also might be interesting: when disconnecting the thermistor, the temp is measured extremely high (sometimes ~4500°, sometimes ~500°). The set temp is still -273.1.

Repetier

Disconnected is same as broken cable so that resistance is high value. So a short is the negative extreme. If you unplug and measure resistance it should then be quite low while it is normally 100k at room temperature (25°C). That would be a good test to test if it is defect.