One of the first things you should if you are having issues is check how well its working by using the M48 command to test it.
If your M48 test results in a value of 0.01 or lower for the standard deviation then the probe is working and something else on the printer is causing issues with the resulting 1st layer. Common issues are: Gantry Being Unlevel, Extrusion Issues with hotend and/or extruder, or mechanical issues.
In our latest firmware (TH3D Unified Firmware U1.R2.X) M48 is enabled by default for ALL machines that we support. Please update to this one before contacting support as it addresses issues present in older firmware releases.
Use commands like this to run 10 probes and get the measurements:
M48 P10 X100 Y100 V2
The output will look similar to this:
M48 Z-Probe Repeatability Test
Mean: 3.450000 Min: 3.442 Max: 3.452 Range: 0.010Standard Deviation: 0.004330
This shows the absolute maximum and minimum measurements the sensor took, the mean (average) of all the measurements, the range between the maximum and minimum, and the standard deviation.
The standard deviation is interesting because it tells us that 68% of all the measurements were within 0.00433mm (one standard deviation) of either side of the mean, and 95% of the measurements were within 0.00866 (two standard deviations) of the mean.
A good probe should consistently give a range in the hundredths of mm, e.g. 0.05mm. A very good probe will have a range in the thousandths. EZABL™ probes typically test UNDER 0.02mm on well setup machines with no mechanical or electrical issues.
If you get a very large range or standard deviation you should look for sources of movement in your Z-axis or the VREF on Z is set too low.
The motor couplers can contribute to excess play and you can consider upgrading them to better ones (like Upgraded Couplers, TH3D sells them), ESPECIALLY if yours are stretched out. Failing that, contact our support for more troubleshooting. If the probe is bad we will replace it under warranty.