I am putting in a new hotend and the printer is telling me the temperature is -30c which im fairly sure it isnt. What could be causing this?
Check the connections, there could be a cable loose or a contact on the board is out of order.
Or a cable is pinched and broken.
all connections are good, no loose wires
If all the pins and connections look good and it’s not shorting out then it’s probably just a bad thermistor. You can check it with a multimeter to see if it’s showing the correct resistance. I think there’s a discussion on here about that if you search for it.
Or just try a different hot end.
-S
Its a brand new Revo, trying to modify the J1 to take revo nozzles with a new heat sink adapter.
If I am not mistaken, the Revo does not use a PT100 as a temperature sensor as the J1 does - it seems to have some NTC inside. This means you are out of luck and have to wait until Snapmaker releases the firmware sources to bould your own firmware - at least if you do not want to build some kind of converter board that reads out the NTC and mimics a PT100.
Hello, have you manage to retrofit the REVO on the J1?
Hey! now that SM release the software would this be possible?
I did get it to fit, now that the firmware allows these changes, I’ll finally be able to make it happen. I’ll see if I can get it done. May need the fan duct to be different to accommodate that change
Yes, it might be possible of you change the firmware to accept an NTC as hotend thermistor.
But you need to solve a second issue, and that is the preamplifier for the PT100 temperature sensor that is located inside the hotend on the PCB where the cooling fan is plugged in. I did not check this option further since my goal is to replace the J1 controller with a Duet 3 6HC, therefore I have no clue what you may need to do with the J1 controller to let it read out the NTC. But I can give you at least one hint: if you add a bridge or a 0Ohm resistor to the empty position on the board (R9, if I remember correctly), then the signal wire from the NTC will be routed directly to the controller board. You may need to unsolder the existing components however since these might interfere.
Another option might be to change the preamplifier circuit that it accepts the NTC and then update the resistance table in the Snapmaker Marlin accordingly. But that need someone who is more capable of analogue electronics than I am 
Edit: ah, @Miles was faster than me 
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I have very little experience with electronics and my only coding experience has been a college C++ class and heavy GPT usage on my recent personal projects
Coding is not needed for that change as far as I can see, but you definitely need to get around that preamplifier somehow… well, I can only cross my fingers for you and wish you that someone here knows how to get that done.
Thx for the advice. If someone can manage the software/ electric part I drafted an adaptor part to mechanically fit the E3D REVO system on the actual Heat sink. So easy mechanical modification once the part is made. Now if the electric can be solve in an easy way this solution could help many of us