Unanswered Temp Drop Error after Auto-Level

Change your startup GCode to something like this:

G28 ; home all axes
G1 Z5 F5000 ; lift Z by 5mm
M565 Z-0.6 ; set the offset for auto-leveling mechanism (yours will be different)
G29 ; run auto-level
M109 S[temperature] ; set the extruder temp and wait

Where it says [temperature] put in the temp you want that model to print at for example

M109 S210 ; set the extruder temp and wait

The problem is two-fold:
1) You have enough cool air moving around the print head that it is cooling down during hte auto-leveling
2) Marlin doesn't maintain the thermistor temp while doing the auto-level dance so it can drift until that is finished. Sure, it doesn't take long, but it can be long enough for temperature on the hotend to drop enough to trigger the TEMP FALL error.

I have one printer that experiences this occasionally and anther near it that does not. This solved it for the one that was having issues.
It is a bit of a pain since you have to edit the startup GCode to change the temperature for that profile, but not a huge issue since those seldom do change.

 

Watch the temperature graph in your software and see if it is falling.
If the Temp Fall error triggered then the temperature did fall.
Could be any number of things from a bad thermistor (a cracked one that reads OK most of the time and flakes when you move it) or a bad connection to the thermistor or RAMPS.

The GCode changes I posted simply avoid it happening immediately when starting. Instead of expecting the temperature to stay set during the auto-level dance that forces it to set the temperature after the auto-leveling and wait -- then start.