Lulzbot mini 1 start problems

Hi! I am using a Mini 1, and it has been fine for a while, good prints, no problems, Now I set it to print, extrude some filament before I press the print start button, to check all is well. When I start the print, it will not lay down and plastic. The extruder seems to fail, the green cog rotates, but when I look at the filament I find the gripper has just chewed out the plastic. So the printer thinks it is printing, but the filament is not being driven through.
I have tried opening the gripper assembly and trying to push the plastic through, but that does not do it. Occasionally I can make it work by pausing, then extruding some plastic using the extrude button, then starting again.
It is as if the filament is hitting a step in the extrusion path and stopping.
I have unscrewed the extrusion head nut and cleared it out so I can see light through it.
Any pointers to what I should try next?

Can you extrude correctly with the print head up in the air well away from the bed? If so then your nozzle may be too close to the bed. What is you Z offset?

I have just put m851 into console in Cura, and get a figure of
Probe Z Offset: -1.75
How do I go about checking whether this is the actual figure?
Thanks for quick response by the way!

Wow that’s awfully low! The washers are 1.5 to 1.6mm thick, your nozzle should be pushing into the bed. Try setting it to -1.4. M851 -1.4 , followed by M500 to save it.

That makes sense, I have been looking around since you made the suggestion and was beginning to think it should be closer to -1.4 or thereabouts. I have certainly grooved up the mat
a bit. I have ordered a new mat from Lulzbot, and will try adjusting it when it comes in. Watch this space and I will tell you what the outcome is.
Thanks for your help!

It won’t hurt to try it now. Probably better to do so now. If it’s extruding and flowing then you should be good to go.

Hi Joe.
I ordered new PEI surface from Lulzbot (mistake) They declined to supply, eventually (I suppose I shouldn’t criticise - probably a Covid thing). I went to Amazon and got an oversized sheet for half the price. Cut down, fitted - all looking good. I reset the Z off-set to -1.4, and all is now well, printing like new!
I am not sure how the Z off-set strayed from it original figure, but it was printing fine up until a few months back.
Thanks for your help, it would have taken me a while to sort it out otherwise!

BTW, the bed-level washers are 1.5mm thick. So the Z-offset should never try to compensate for more than 1.5mm.

For materials where I want a bit of “squish” on the filament (e.g. PLA) you might use a Z-offset value of say… -1.3

For materials where I want a bit less “squish” (e.g. PETG should be gently laid onto the bed and not squished onto the bed) I raise the nozzle a bit and use a Z-offset of something like -1.2.