Thank you, I re-calibrated, and it printed first time, so set another print on and failed again!!
There are many causes. You already did the lift switch calibration. Really the first step to diagnosing this is to watch the process carefully to determine if it's an electrical issue or a mechanical issue. So please do that and report back.
1) Surface detected too early. As it does the leveling it never even touches the glass. This indicates electrical problem such as nearby interference. Do the leveling sensor test in maintenance and report back on the number it displays (< 8 is a pass).
2) Surface detected too late. The test relies on the capacitance changing. The surface isn't detected until the glass moves down without the nozzle moving up. Visually you see it touching the glass and still moving down more on one core than the other and it's usually obvious. It could be the springs are too tight on the print bed or too loose on the core but this is very rare.
3) Cores are truly different heights. This more common than you might think. They can vary. If you look in the log file you can see what was measured. The cores are supposed to be 1.5mm different height nominally with an error of only 0.7mm allowed. It could be a bit of gunk on the nozzle tip that wasn't hot enough to squish flat during the procedure. This is extremely difficult to measure on the core itself with a micrometer as it's the height from the tip of the core to how it seats inside the print head so to the 45 degree angled area of the core just above the heat break. So if it's issue #3 I'd just try a different core on the right core. Or left core. Sometimes the lifting switch doesn't function so it measures the same nozzle twice which is of course a problem. This is the easiest of all things here to fix.
For some reason #3 used to be the main issue but in the last year or so #1 seems to be most common and it has to do with the electromagnetic interference from the front fan.
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One I often forget to check is if the lift switch is correctly calibrated.
I also like this list of troubleshooting tips:
Good Luck 💪
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