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Just installed a j head but the thermistor shows -20c, does that have to do with the heater cartridge? do i need to change a setting?

Bill

I discovered that the bed doesn't heat up now, even hooked up the old head, still shows -20c. none of the red lights come up on the Rambo and I checked all three fuses....... stumped

Nothing will work until you get a reasonable reading from the thermistor. The firmware goes into a fails are mode.
The odds are it's a wiring issue, start by measuring the resistance of the thermistor at room temperature, it should be about 100KOhms.

I'm having a similar problem with my unit. I got it all together and the hot end reads -20c, bed reads room temperature. I think I might have messed up the leads on the thermistor but can't tell because my multimeter doesn't go that high, it just says open. Does Repetier error when either thermistor is reading way off or just when the hot end is reading off? Can I swap the thermistor leads to prove the hot end one is bad?

Yup, this is a good way to test the components and eliminate thermistor issues being the cause.

Mine started working, must have been a bad crimp on my disconnect

glad you figured it out. where did you get your jhead?

A buddy gave it to me, I think it came from maker shed?

Bill