Inconsistent first layer caused by unexplained Z drift  

Inconsistent first layer caused by unexplained Z drift

Hello!

A friend of mine got a brand new Prusa MINI+ (pre-assembled, not kit) just a few days ago, and we simply can't get a consistent first layer. We've now spent close to 20 hours troubleshooting the thing, to no avail. We've checked every nut and bolt, squared the printer, reseated every connection everywhere, and run about a dozen first-layer calibrations. We even reflashed the firmware just to be extra sure, but nothing helps.

Then we connected it to Pronterface and ran the G29 command to perform bed leveling. The results are wildly inconsistent, and drift towards higher and higher Z values with each leveling!

      0       1       2       3
0  +0.086  -0.156  -0.146  -0.119
1  -0.062  +0.019  +0.033  +0.204
2  +0.160  +0.096  +0.176  +0.385
3  +0.100  +0.225  +0.495  +0.680
---
      0       1       2       3
0  +0.403  +0.217  +0.179  +0.205
1  +0.270  +0.470  +0.516  +0.706
2  +0.629  +0.662  +0.738  +0.925
3  +0.799  +0.966  +1.119  +1.328
---
      0       1       2       3
0  +1.633  +1.477  +1.492  +1.515
1  +1.553  +1.651  +1.722  +1.881
2  +1.790  +1.812  +1.889  +2.094
3  +1.756  +1.957  +2.110  +2.310
---
      0       1       2       3
0  +3.085  +2.932  +2.910  +2.852
1  +3.039  +3.146  +3.249  +3.367
2  +3.385  +3.348  +3.422  +3.599
3  +3.410  +3.642  +3.796  +3.996

The values, they're all over the place!

For comparison, here's my perfectly well-behaved Prusa MINI+:

      0       1       2       3
0  -0.627  -0.431  -0.202  -0.046
1  -0.612  -0.272  -0.061  +0.112
2  -0.588  -0.302  -0.069  +0.156
3  -0.724  -0.395  -0.077  +0.222
---
      0       1       2       3
0  -0.624  -0.428  -0.198  -0.039
1  -0.613  -0.276  -0.061  +0.118
2  -0.586  -0.301  -0.069  +0.167
3  -0.739  -0.411  -0.086  +0.209
---
      0       1       2       3
0  -0.639  -0.443  -0.211  -0.054
1  -0.629  -0.291  -0.076  +0.104
2  -0.603  -0.316  -0.083  +0.152
3  -0.746  -0.414  -0.096  +0.199
---
      0       1       2       3
0  -0.611  -0.420  -0.184  -0.023
1  -0.599  -0.264  -0.056  +0.126
2  -0.570  -0.286  -0.055  +0.176
3  -0.723  -0.398  -0.076  +0.217

And I plotted them visually so you can see how crazy this is:

My printer on the left, his on the right. It's no wonder he can't get a consistent first layer when the printer can't decide what shape the bed is.

These results were achieved without homing the respective printer between G29 calls, and with everything at room temperature.

So, to the question: What could be causing this? Is it likely to be just a broken SuperPINDA, or could there be something else going on? Does anyone have ideas on how to troubleshoot this further? I want to reiterate that this printer is brand new, and we were very careful during assembly. I'm not sure how a broken SuperPINDA could cause the Z value to drift off into the stratosphere like that...

Any help at all is appreciated. Thank you for taking the time to read.