When heating the hotend/hotbed temperature fluctuates significantly (Premise: temperature can reach set value)
2.1 The pre-set PID temperature of the machine is not suitable for the local environmental temperature.
2.2 Insufficient heating power.
3.1 Run the automatic heat bed/nozzle PID. (Control - Temperature - Auto PID)
3.2 If you have a multimeter, measure the voltage on the mainboard hotend/hotbed ports when heating. Please refer to the tutorial https://youtu.be/pa26JHvucrs