Can anybody tell me why I’ve started getting negative results from my auto levelling? Out of the blue the nozzle is being rammed into the bed on the back four points. I haven’t touched the firmware since setting it up the other day.
Hmmm. Does recalibrating correct it? I always get hotend to temp and let it sit 5 minutes before Setting it or running autolevel. All hotends grow s little from cold to hot… it’s a fraction on a mm but it does expand. Just an idea.
Brook
I’ve also heard people say running the heatbed affects the probe. But Tom did a pretty thorough test on a few models of probes and said it wasn’t worth worrying about. Not sure if that is true for all.
I take care to put my offset on probing to an area where not screws or rails are directly underneath. In some circumstances, an induction probe will read higher in one corner if it proves there over more dense metal. Then it appears to be more tilted than it really is. Ruins your print. I also stay away from the sides 20mm for safety. If you are probing right on the edge, it affects the reading as compared to over a solid surface w a wide path around it also solid.
I’ve only just started it when I got this result, hot end and bed are cold but I get the same when it is up to temp. I thought it might be the inductive sensor but it only does it at the back end. I will try and upload the firmware again tomorrow to see if that helps.
I ran M851 and it gave the correct z offset the ran G28 followed by G29. I was wondering if I had done something to the firmware but I doubt even I could make it do this.lol
Sorry no help on the neg values, but ironically I started having some difficulties with mine this week as well. It’s the blue end, 4mm proximity sensor, and I found that I needed to lower it by a mm or so to get a correct z height. This after printing hundreds of items with the previous config, but now it’ll make periodic contact with parts as it traverses the bed because it’s so low. I ordered the 8mm probe but didn’t realize the barrel was so much bigger so now I need to make a new probe mount for my Titan Aero. These printers are like working on old air-cooled VWs…it just never ends.
@Tim_Sills yes but working on an old Vw is sooooooo much more rewarding 
Ha! The VW mechanic in me agrees! Those sensors (china) can die on you… we’ve sourced the best we can find. I don’t carry the blue anymore. Ours are orange. Funny how source can matter even if it reads the same on amazon or Ali.
Thanks for the feedback @Brook_Drumm . I had suspected it was failing in some fashion or another. I have one of the orange tipped ones in front of me right now to install.
I haven’t had chance to check mine yet but I might order an orange tip one anyway. Anybody got a reliable source for one that delivers to the UK or do I just go with the aliexpress special again.