Thermal
This is about all sorts of thermal problems.
First there is that cute silicone sock that is over the nozzle on a lot of printers. It does two things, in theory. First, if you have a huge blob of filament on the hot end the idea is that it will be on the sock and you can get it off. I’ve never seen that work.
The second thing it does is help keep the temperature of the nozzle constant. When it’s on it provides some protection from breezes changing the temp. The insulation it adds helps keep the required energy down.
Your printer will probably work without it. Your printer will probably work better with it.
Now, my printer has thermal runaway, my printer keeps cooling down while printing, other temperature things.
First let’s look at how temperature works on a printer. The printer has no idea what the temperature is. There is probably a thermistor someplace, in the aluminum block the nozzle is screwed into or on the nozzle. This has a bit of metal that changes resistance as its temperature changes. The motherboard gets this resistance and converts it to a temperature. A little bit of current comes out the motherboard and through the bit of metal. The voltage drop gives the resistance.
If the thermistor has a broken wire, or it is unplugged, or it is broken there is infinite resistance at the motherboard. If some printers see infinite resistance they show a temp of -17, some say 0, some say something else. If you see a temp like this find out where the open circuit is and fix it. The wires are thin and break easily, especially if you had a blob of plastic and took it off. The thermistor, the little glass bead on the end, cah break. There can be a problem with the plug into the motherboard. I’ve never seen it, but a problem in the firmware could cause bad temperatures. Again, I have never seen this happen.
The other problem, much less common, is a short. It gives a wrong temp and probably never changes.
Most printers show 4 temps. What the bed and head are and what it wants the bed and head to be. Yeah, just like the head there is a thermistor in the bed and it gives the temp.
When you are printing the gcode sets the temps for the head and bed. These can change, for a temp tower they have to. If you can not see differences in the tower, watch as it prints and be sure the temp the printer wants changes.
The heaters for both the bed and hotend are simple. They are on or off. No part on to keep the temp up. When you start the heater comes on and when it gets to the desired temp it shuts off. The hot end keeps moving around the desired temp. It heats above it, then cools down below it, then heats up again. A PID tune lets the printer figure out how stuff works. As the hotend is heating up if it turns the heater off the temp will keep going up for a bit. And if the hotend is cooling down and the heater comes on the temp will drop a bit before rising again. The PID tune gives the printer a chance to see these things so it will turn the heater off as it gets close to temp and turn it on early if the temp is falling.
The temp will never be entirely flat. There are always ripples. A PID tune makes these ripples smaller. If you change stuff run the PID tune again.