The magnet to trigger the sensor is glued on the bottom of the print platform. The z-sensor points towards the magnet. Once the magnet with its magnetic field gets close enough to the sensor when the print platform moves up, the platform stops moving (-/+ the offset that you calibrate when setting the z-gap).
The z-sensor is positioned between the print jets and fans.
The temperature in that area can vary with the jets on or off and the fans on or off,
additionally the 6 fans (if you have a trio) can produce a different magnet field when they are on or off.
So all this "noise" could have an effect on the repeatability of the z-sensor.
Since I repositioned the z-sensor and magnet to a less "noisy" location, the z-gap has been very consistent.
The sensor repositioning kit came with the heated printbed, but you can get the z-sensor kit by itself from GRM products.
It installs easily without modifications to the current z-sensor setup. The current sensor just gets unplugged and the new sensor from the kit plugs into the connector instead.
This little modification is well worth it.
Also, make sure to check the "Print bed leveling aid" post, for easier print bed leveling.
Just as a reference, here are pictures of the location for the X and Y homing sensors:
In general, this is what I do before printing:
- Select the "Home" button. This will move the print jet behind the print bed so you can do step three.
- Make sure the jets don't have any (not even a tiny piece at the tip) of filament left stuck on them otherwise it can affect your z-gap measurement.
- Hit the "z-gap" menu button and check or adjust the z-gap with the up/down button.
- Then print.
(Note: I am not affiliated with GRM products and I have to buy my stuff like everybody else, but I gladly promote companies with great products and services that make my life easier).