When there is a height difference between the two extruders; support material will not properly attach to the printed part or will melt into the printed part, the print sticker gets torn up, the object never succeeds in printing the base layer, or the object is repeatedly ran into by the lower of the two nozzles causing a failed print.
STEP 01: Loose the grub/set screws that secure the extruders in vertical position, you can locate them behind the fans as shown in the photo below. Make sure you turn the printer off before you attempt to loosen the grub/set screws behind the fans on the extruder. You just have to loosen the set screw you don't need to continue unscrewing it. You need a 2mm Allen key to loosen the grub/set screw.
STEP 02: You can use your hands or pliers to grip the block the hot end/nozzle passes through and gently pull straight down. Be very careful to pull straight down and not to bend or torque the block or hot end shaft in an awkward direction. If the heat was on recently you should use protective heat gloves for your hands to avoid burning yourself.
STEP 03: With the machine off manually move the print carriage to the center of the printing bed.
STEP 04: Turn the machine on and use your software to Home the Z-Axis - there will be a button in the section of your 3d printing software that gives you manual jog control over the printer. This button is usually symbolized with a picture of a home or is marked with the text "Z Home". Remember we just want to home the Z-Axis and not the X or Y-Axis. When the bed raises towards the nozzles you have lowered and left unsecured the bed will push the nozzles upwards to an equal aligned vertical position.
STEP 05: Turn the machine off and now tighten the grub/set screws located behind the fan blades on the nozzles to secure the extruders in perfect alignment with each other.
STEP 06: You must now level your print bed glass. Please consult the "Bed Height Calibration Solution Article".
Below are examples of acceptable and unacceptable nozzle to glass bed gap and their symptoms.
Too much distance will cause the filament to extrude into the air. This will not adhere to the bed.
The tip of the nozzle is adding slight pressure to the top of the filament, greatly increasing how much filament is securely sticking to the bed and subsequent layers.
The filament is not flowing properly, which can cause retrograde extrusion. Layers are predominantly choppy and short. Continued printing in this manner will likely cause a clogged extruder tube and/or nozzle.