Easy to test. Go to the diagnostics menu and choose the item disable cartridge check. When the printer says ready, diagnostics mode, open the toner door and swap the cyan and magenta toners. Close the door and run the same page again. If still magenta lines, the issue is the toner cartridge and if original hp, return to where you purchased for refund or exchange. If you now have cyan line, then the slot has an issue and you will require service.
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