I have a long white line from end to end of the print. The line is on every print. The ribbon is now all used up. If I replace the ribbon will the problem go away? Is there anything else I can do? I don't want to buy another ribbon for $40 if that line will never go away. What to do ?

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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Line sensor calibration has must to do if Line Sensor is disassembled or replaced.
In the Service Calibrations submenu, scroll to "Line Sensor Calibration" and press Enter.
Follow the snaps for replacing the line sensor...

I had this same problem and none of the printer services (clean printhead, align printer, calibrate linefeed) fixed the problem. After trying every solution that I found on the internet with no success I was able to fix the problem by cleaning two critical parts of the printer.
1. Turn off the printer and open it as though you are going to change the printheads.
2. Towards the back of the printer you will see a thin clear plastic ribbon that has very small black lines on it. Very, very, very gently clean this ribbon. I used a lint free cloth and some glass cleaner. Spray the cleaner on the cloth and slide the cloth against both sides of the ribbon. Do not pull on the ribbon. This ribbon is used to tell the printhead how far it has moved from side to side.
3. Next locate the silver bar that runs across the back of the printer. This bar is used to feed the paper through the printer as it is printing a document. On the left side of the bar you will see some plastic guides that hold the bar in place. On the outside of the plastic guides you should see a clear plastic disc made of the same material as the ribbon above. This disc also has very small black lines on it. Again very, very, very gently clean this plastic disc. This disk is used to tell the printheads how far it has travelled down the page. This is the one that was causing the problem. The dust or dried ink or whatever was on the disc was causing the printer to incorrectly calibrate the linefeed and was causing the white lines to appear on my documents.
4. Use the "calibrate linefeed" function of the printer services to recalibrate the linefeed and the white lines will hopefully be gone.

PR-01724 Ribbon Break

The print ribbon is broken.

Possible Cause 1: The print ribbon has broken or is not installed.

Solution 1: Trim the broken ribbon evenly and tape the cut end of the ribbon to the ribbon on the take-up spool. Install the ribbon cartridge.
NOTE: Do not switch take-up spools when fixing.

Possible Cause 2: If the ribbon type is K or KT, the print ribbon is at the end of the roll.

Solution 2: Load a new roll of monochrome print ribbon.

Technical issues:

•If you have recently changed edge-to-edge settings using the SmartDriver, printing might be too close to the edge of the card. Change edge to edge settings.

•You might have moved the print ribbon while the printer power was off. The next card will print correctly. The printer is designed to allow you to fix problems while the printer power is on.

•If the print ribbon is stuck to the card, the Intensity setting might be so high that printing causes the ribbon to stick to the card. Lower the Intensity or power setting.

•The card design might be longer than the card size. Change the card design to be less than or equal to the card size in the long direction (3.37 inches or 85.60 mm).

•Printing might not be positioned properly on the card. Change the "Left/Right" position of printing (using the SmartDriver) so the printing begins and ends on the card.

The Zebra P110i printer is an Edge-To-Edge card printer, which means it will print on the card closest to the edge as possible, however there will still be a slight line. Although a 2mm line is too big, should be no more than 1mm, there will still always be a line.

There are some advanced EC (end card) settings that could be adjusted but if the print goes too close to the edge, the ribbon will get cut.

-- The Pros at www.DiscountIDStore.com

If it's not the ribbon, then it would most likely be the printhead assuming there's nothing else blocking the printhead or ribbon.

Could be down to tension on the ribbon roller/spool. Check to see if you have pulled out the end part of the spool (black bit) that should ensure there is back tension on the ribbon.

If the machine has mainly printed 6x4,When you
go to print 7x5 the edges of the head and rollers
tend to be dirty and will leave the two stripes along
the edge of the prints.Use the cleaning ribbon and the
sticky pad supplied with a new box of paper to clean the machine.Depending on how bad it is it may take a few try's.To do this switch the machine on and enter the control panel by tapping twice on the top left of the monitor- It will ask for a four digit code - Uless it has been changed this will be 9999 click
Enter/Ok - then click on the top far right ikon and
the screen you see will have a greyed out ikon
bottom middle - Now replace the print ribbon with
the white cleaning ribbon supplied with the machine
and the greyed out button becomes active - Click on
this button and the machine asks if you want to clean
it click Enter/Ok - About 10" of paper will then be
driven from the machine place the sticky pad as per
the drawing on it onto the paper and press Enter/Ok
- The paper will move in and out a few time and then eject a big piece and some small pieces of paper - Then remove the white cleaner ribbon and replace with the print ribbon and press the green return button
in the bottom right of the screen to return to normal use.This should be performed every two - three boxes
of paper.

If printing with ribbon, check to see if you have ribbon burnt onto the printhead. If so, lower the temp of the heat on print. Or you can speed up the print speed.
I have seen this happen alot when the ambient temp is low, and the printer tries to make up for the differences in temperature.
Check the hed first..

Question, when the printer prints is there an image of the data on the ribbon when it spools onto the ribbon takeup spool? If so, the ribbon may be installed backwards. I'm sure you know what you are doing but one must check even the smallest detail when trying to solve these strange problems. If the ribbon is installed correctly and there is no image of the data on the ribbon you may be dealing with a bad printhead and the printhead needs to be replaced.