For ZM400 and S4M, Has anyone had any luck with printing a white ribbon and a black or dark label without covering up the ribbon sensor with electrical tape? Isn't there a setting that can be adjusted?

Try removing the ribbon cassette and pre-tension it. There is help in the link.

https://www.google.com/search?q=Zm400+is+printing+white+lines

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Perhaps your darkness is set too low, or your labels are not coated to accept thermal transfer printing. Let me know if this helps!

I am assuming you mean misalignment of lable top or start of print moving on label.

check sensor for lable debris.

if OK then perform media calibration:

Enter menu and scroll to ADVANCED

press enter and use password 9999

scroll to MEDIA and RIBBON CALIBRATE

Remove a few labels and remove ribbon

Press right arrow to continue display CALIBRATING PLEASE WAIT.

When complete reload ribbon and labels press right arrow printer will feed approx 39"

printer should now print OK.

It is the same loading procedure for an S4M or ZM400. Here's a link:
http://www.zebra.com/id/zebra/na/en/index/products/printers/industrial_commercial/s4m.html

The ribbon has wax just on one side. So, if nothing prints on the label, first thing I'd check if the ribbon loaded with waxed side facing label.

turn printer off, press pause buton, tur on, wait 5 secunds and relese the button. He goin to print the test label, Pause. check Darkness seting, ajust if necessery. don't forget to save. To go out from the test, turn printer off and on again.
the ribbon is on the correct way?
The labels are Termal Transfer and you don't have ribbon in the printer.

By gaps, do you mean a "void" area ? Over time, the elements on the printhead will wear down and finally they won't transfer heat to the ribbon. This causes a gap or void area. Usually the elements go out one or two at a time. If the area that is worn out is supposed to be printing bars for the barcode, this will cause the scanning problem you are experiencing.

The best way to see if elements are worn out is to design a label that has a thick solid black line across the label or a solid black box. You will see white vertical lines in the areas where elements are worn out. Another method would be to use the widest and longest label you have and print a 'pause" test label. This is done by turning off the printer, pressing and holding on the PAUSE key while turning on the printer. Hold the pause key for 10-15 seconds and when the printer is done with it's power on self test, it will start printing out a test pattern label which includes a nice black area where you can look for the white vertical lines.

Hope that helps !

I haven't tried yet but after doing extensive reading feel that the problem is in the ribbon type. I thought it was a resin wax blend but it was one that I had for awhile without any label on it and I'm thinking it may be just a wax and that is why it works on white labels but not on the poly or mylar.

I was having same issue printing from our AS400's as well.

Issues was resolved by installing latest Zebra ZM400 firmware.

We were on ver V53.16.3Z but upgrading to V53.16.4Z seems to have resolved it.

Good luck.