Prusa i3 Extruder Won't Heat Up

Just as the title says, the hotbed works fine and it'll go all the way to the desired temp. But the extruder, the important bit from what I understand, doesn't do diddly, just cruising at room temperature no matter what I do. I've tried unplugging the thermistor and plugging it back in, but other than that I'm not much of an electrician to see if the thing is actually getting power. Everything else works fine, except the heating for the extruder. Any tips? I just finished building this thing, and please, for anyone trying to help me, speak to me like I'm a complete novice because I have no idea what I'm doing when it comes to 3D printers. Computers are my game, printers of any kind are my bane.

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Hi,
printer problems like this one are also logical like computers;
There is a safety features within the firmware which stop the hot end from heating if a fault is present. Able to use a Multimeter?
if so place it on the volts setting, turn on the hot end set temp to 200 and place probes across the hot end wires, you should get a voltage reading ~12 v hopefully you will get one. Now place it on the ohms scale and with the power off, place the probes on the heater cartridge wires, you should get about 3 ohms. If no value heater cartridge is dead, look at getting a replacement and that's the reason it won't heat up the hot end.

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Welp, I was hoping I wouldn't have to do that because I lack a Multimeter, but so far all the signs are pointing at it. What happens if I get a multimeter and it reads fine?

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The multimeter doesn't need to be a top end one just basic/mid range one will do, also one with continuity setting will be helpful to find a break in wires, here is wiki page https://en.wikipedia.org/wiki/Continuity_test

If there is an Ohm's reading from the heater cartridge then it's fine and you will need to dive a little deeper as it part of a simple electronic circuit.

The hot end circuit requires power (volts), a Mosfet (aka relay), a PWM signal from micro-controller from a pin defined in the firmware file, heater cartridge and the thermistor (sense temperature). Also take a look at the build instructions for the printer the prusa i3 is quite well documented and supported upon the web. You could just buy another heater cartridge as it's unlikely to have 2 faulty ones.

As you have stated that your printer's controller shows room temperature then I believe the thermistor it's working. If you disconnect one of the wires from the board or sensor. It should flag a heater fault. This a firmware safety feature which breaks the hot end circuit so the heater doesn't overheat the nozzle and wiring.

Also did you check the firmware settings? By using the Arduino IDE and opening the configuration.h file. The i3 most likely uses marlin (that's the name given to the firmware) It reads the data, settings from the given USB connection and tells the stepper motors to move. This you tuber Thomas Sanladerer gives an overview of firmware basics

He also does other videos which will help you get the best from your printer, once you get know them, they are quite simple but frustrating when they developed a fault.
If the incorrect board is configured in the firmware then that will cause the fault .
There could also be break in the power wires or a bad connection which is breaking the hot end heater circuit thus not heating the hot end
To help more you will need to upload photos and details of printer's controller board so able to correct information.

Let us know how you get on

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Welp I figured it out and it was my fault. I didn't do a good job stripping the wires before plugging them into the extruder port, stripped them with an actual tool and it lives!

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Cool, a nice cheap and easy fix; One must provide a path for the electrons to flow.
Enjoy using the printer

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I've had problems on my i3 MK2 where terminal block (the part where the bed plugs into the main board) for my heatbed basically melts and ultimately fails and I have to replace it.  I would guess that that could happen to the extruder terminal block too.  It doesn't seem like that's your problem but maybe something to check.  

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Also a red light comes on when I tell it to preheat for PLA. The hotbed still heats up but the extruder refuses to cooperate.

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