There are two issues to check. (1) is the printer able to ‘home’ each axis correctly, and (2) is the bed-leveling system getting adequate electrical continuity for the leveling process.
Each of these are actually pretty simple things (the printer is, in some ways, simpler than you might suspect… very simple electrical continuity is used to determine when an axis has hit it’s travel limit and a similar system is used to level the bed. Either it conducts or it doesn’t. It can be as simple as a wire that came loose.
This is all based on ‘homing’ the printer (the G28 command) … which runs each axis until they hit their end-stops.
You can test the printer by going into the “Console” (you’ll find a button to open the console on the “Monitor” panel of Cura). This is an interactive terminal window that lets you type direct commands to the printer.
In the terminal do a ‘G28’ command (home all axes). Watch for any error messages.
Then test out a few moves such as: G0 X100 F2400
That will move the X axis to 100mm in from the left of the build plate and it will travel at 2400mm per minute (40mm/sec).
Then test: G0 X0 F2400 (this should move the X axis back to the left edge of the build plate)
Test: G0 Y100 F2400 … and also test G0 Y0 F2400
Basically you are testing to see if the printer will obey your move commands. If may refuse a move on an axis if it thinks the axis hasn’t been ‘homed’.
The Workhorse should home itself, heat the nozzle to the ‘soften’ temperature for the particular filament (this varies based on the filament profile), it will back out a little filament, then cool to the ‘wipe’ temp (usually 170°C but this is also in the filament profile), then perform the wipe operation on the pad, then cool to the probe temp (usually 160°C) then probe each bed corner starting with the front left washer.
If it fails it means it didn’t get electrical continuity when the nozzle touched the washer. This could be many reasons.
The main reason is a dirty nozzle – make sure it is clean. You can heat it up and use one of the red scrubbing pads to clean it. I usually use the tweezers to pull anything clean then give it a scrub (these days I converted from the hardened steel nozzle, to an E3D Nozzle-X. Nozzle-X is also hardened steel but it has a coating that makes it difficult for any filament to stick to it. If anything does manage to stick, it wipes off easily because of the coating.)
There can be other reasons for a failure such as a disconnected or broken wire.
Grab your multi-meter and test out the ohms on each washer as well as on the head. Nothing should read more than 1Ω.
For example, I’ll put a probe on the calibration cube while putting the other probe on each of the four bed leveling washers. Make sure nothing reads above 1Ω.
Also put a probe on the red ‘zero-sense’ wire on the AeroStruder. This is the red-wire attached to the lower-right corner of the aluminum heat-sink. Put the other probe on the tip of the nozzle and make sure that reads less than 1Ω.
My printhead had a filament leak through the heat-brake. I wish I had a diagram to show but … your heat-block on the print head has a hole drilled and tapped completely through the block from top to bottom. The heat-brake tube threads down into that hole from above. The extruder nozzle threads up into the hole from the bottom. The bottom of the heat-break and the top of the nozzle MUST form a tight seal … no gap. If they aren’t tight, filament leaks through the gap, into the threads and eventually starts oozing out of the top of the heater block. If you inspect the heat-brake gap (the gap between the hot-end and the cold-end of the extruder) it should be clean. If you see filament in that gap, you’ve got a leak and it needs to be disassembled, clean, and re-assembled to have a tight fit.
Anyway… in my case, the “leak” means that the filament acts as an insulator … it works its way into the threads and instead of metal on metal, you have metal with polymer between it … so it barely gets any electrical contact. This will result in poor probing when it tries to level the bed.
It can be difficult to see into that gap with the fan-duct installed. The cold-end of the extruder is held in place with four screws (one in each corner around the heat-sink). These are long screws. The bottom two screws also hold the fan duct in place. If you remove them then you can pull down the fan-duct (careful not to strain the fan wire because the fan is attached to the duct) and easily see into the gap. BTW, these two screws are not the same length, so remember which one was on the left vs. right (although it’ll probably be obvious when you go to put it together and one is too short and the other is too longer… just swap them).