Z9 Drop Detector Calibration issue

[indipix]

There was a lot of horizontal banding in the prints. The print heads were showing OK. On doing the Drop dectector Calibration, there were errors. So the HP service replaced the whole drop dector assembly. After a few ok prints the banding stated again. On repetedly dong the Drop Dectector Calibration, it would fail sometimrs and be OK sometimes. when it showed Ok, the printing was fine and when it failed, the banding in the prints came.

Does anyone have a solution for this, because even the HP service team cannot provide a logical answer to this issue. I have attached images of banding, OK print and screen shots of the OK/Failed Drop Dector Calibration.

[Valsimot]

How old are a printer and the printheads? You might need to replace some of the printheads to fix banding.

Drop detector detects if any of the printhead nozzles are defective, and then that information is used to substitute defective nozzles with the good nozzles, but sometimes that doesn't work because of printhead problem.