Start here — before replacing any parts
If each successive shipping label lands lower, first correct label size and calibrate the gap/black-mark sensor.
Confirm the XP-420B hardware interface and the paper/sensor type in your printer.
Manual scope: Confirm gap or black-mark media type
Do these checks in order
Follow the steps below and keep a record of what changes after each one. Stop when an actual cause is confirmed.
Measure the actual label WIDTH and HEIGHT. Set those values in your shipping program and the XP-420B driver; remove accidental margins/vertical offsets.
Load the roll with the printable face and paper guides in the positions shown by the manual. Close the printhead firmly.
Open Xprinter Diagnostic Tool, select the working interface (USB by default when actually connected by USB), choose Calibrate Sensor, then choose Gap, Black Mark, or Continuous to MATCH your stock, and select Calibrate.
Print several consecutive identical test labels. Note whether the start point stays fixed or creeps down again; do not immediately replace the sensor.
If the problem is gone
Position stays consistent across multiple labels: the size/media sensor calibration resolved the progressive drift.
If the problem is still there
Only the computer-generated label drifts while the printer self-test is normal: investigate the shipping app/page size and driver; if self-test also drifts, recheck sensor/media path.
Which cause matches your result?
Use these more specific checks only when the earlier steps point to that possibility.
01Label height or vertical offset is wrong
Compare physical label dimensions with the printer driver and shipping software settings.
Correct label height and any vertical offset setting.
02The gap sensor needs recalibration
Check the selected media type and whether the label gap is correctly detected.
Run the appropriate media-sensor calibration using Xprinter Diagnostic Tool.
03The drift continues on a test label
Test with the supported driver and a simple label, noting whether each new label starts lower.
Review the sensor and media setup before considering hardware replacement.
How can I tell it is actually fixed?
Print at least five consecutive labels of the same design and confirm the cut/start position remains consistent.
Before buying a replacement part
No replacement sensor is justified by offset alone; identify the fitted sensor assembly first.
Where this guidance comes from
Primary reference: View the manufacturer or manual reference ↗
Applicability: Confirm gap or black-mark media type. Source evidence: Matching model owner question.
The instructions are original English troubleshooting explanations derived from the listed technical facts; they are not a verbatim service manual. Do not apply them to an unverified revision.