@kgbengt When you get erratic readings, do you mean the probe reading reported in DWC moves around (say, +/-5), or that you get sporadic 999999 readings?
Please report what size coil you are using (12mm or 15mm), and the response to M558.1 K# and M558.2 K#, where # is the probe number.
I've been doing some testing recently (still ongoing sorry for the slow update). The setup is quite sensitive; I can configure it to produce sporadic 999999 readings by setting the drive current to 16 rather than 15 on the 12mm coil.
A few notes on mounting:
In theory the 12mm coils are higher resolution than the 15mm coils but we need more testing to confirm which is best so we provide both (in progress: very generally, I find it easier to get sporadic 999999 readings with the 12mm coil)
Make sure there is no metal in the 30mm ABOVE the coil, or it will pick this up and give false readings
Mount the coil 1-2mm above the nozzle.
Route the SZP ribbon cable away from the coil, ie make sure it doesn't go over the coil
Ensure that the ribbon cable is FULLY inserted into the tiny connector, at both ends. Try reseating this if you continue to get erratic readings
I use M2.5 countersunk screws (often supplied with smaller fans) to mount the coil, which stick out about 0.7mm. M3 countersunk screws stick out a bit further. There are M3 screws with low profile heads, usually between 0.8mm and 1mm; search for "m3 ultra low profile thin head".
Configuration and calibration is a little vague in the instructions, but mostly correct (I've just added that reading offset needs to be set in config.g as well as drive current); I have some notes on that, but need to check and verify them. Briefly:
My coil is mounted 2mm above the nozzle, I have G31 Z2 set, so it is 4mm above the bed when it triggers.
The part that needs testing is the height to calibrate the drive current. I tend to do this at the same height as G31 is set, ie Z2. The instructions aren't very clear on this at the moment.
drive current AND reading offset should be set with, for example, M558.2 K1 S15 R101133 (use the numbers you get from the drive current calibration)
Note that the current instructions tell you to check for 'sensible readings' (ie not 999999 when far from the bed) BEFORE reading vs height (M558.1) calibration. I've found I can get sporadic 999999 AFTER this calibration, but only when further (ie Z5 and higher) from the bed. I'm trying to narrow down the factors that cause this, but it seems to be setting the drive current too high.
reading vs height calibration is pretty much as in the instructions.
Let me know if that helps.
Ian