Diagnosis
The tool is operating correctly. The XWT15Z is a standard impact wrench, not a drill/driver with a separate impact mode. The hammer mechanism activates under load.
How to Fix It
This is by design. Use the variable speed trigger to control RPM. For precise, non-hammering work, use a low RPM setting and very light trigger pressure. The impact will only engage if the fastener provides sufficient resistance.
If That Didn't Work
If impacts are occurring violently with almost no load, the hammer springs may be broken. This requires service.
Heads Up
Do not 'feather' the trigger for extended periods to avoid impacting. This overheats the motor. Use the right tool for the job.