HP4550 Quality issue: How to identify reason

[elkooo]

Hi there,

I have a HP 4550 Color Laser Printer.
Lately it startet printing the cyan color unevenly. On the right side of Images, the cyan color is much stronger, starting with kind of a gradient (about 1" wide).
The issue is only visible on images - when no toner needs to be applied, there is also no cyan. It seems like the saturation of cyan color is uneven.
Other then that, the quality of prints seem to be ok.

All toner levels are >50% (yellow was replaced recently). Also, all other consumables (drum kit, transfer kit, fuser kit) are at over 60%. All installed material is genuine HP.

I have no idea where to start. Can you help?
I can also print a test image for identification of the problem - but did not know what to choose - perhaps you might suggest something?
Any help is highly appreciated! Thanks alot,
Best Regards
Frank

(edit: calibration page looks like this... the cyan-saturation on the right is clearly visible)

 

[Claremidott]

when we had this issue in the past we found the powder was to solid inside the toner. it needed to be loosened up with a gentle shake