While temperature variations are unlikely to result in a failed 3D print, they’re unsightly.
Signs of temperature variations:
Variations between layers will be visible. You can diagnose this from layers with uneven extrusion: [Under Extrusion] and [Over Extrusion].
Explanation:
These variations in layers are often a result of temperature fluctuations. Best would be to try a few tethered prints while running a monitoring software. For example the free and open source Pronterface to graph the exact temperature at the nozzle during printing.
How to fix:
Try some PID Tuning. If you’re not familiar with PID tuning, YouTube has tons of tutorials on how to do PID tuning on various printers.
Check the heater cartridge leads for any breaks or lose connectors.
Check the Thermocouple leads for any breaks or lose connectors.
On E3D HotEnds, check the black Molex connector for the thermistor, or better yet, remove that connector and crimp/solder the leads instead.