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Have you checked the "Draft" or "Economy" setting for the printer? This results in the printer skimping on toner to produce barely acceptable printout good only for previewing a printout. Below is an example for an HP Laserjet 1100 showing the "Economy" setting (highlighted in green) disabled:
If the printer is not in "Economy" mode, then the next most likely issue is that the high voltage is failing to transfer all the toner from the drum/belt to the paper. This can be checked by looking for excess toner inside the casing and "ghosts" further down the page. Unfortunately, confirming that possibility is an issue for an HP technician with the service manual, because it requires special equipment and the high voltage is potentially lethal without proper precautions.
Another possibility is a worn drum or belt. However, all modern laser printers warn when the drum or belt is reaching the end of its life, so this is not as likely as a high voltage issue.
Check the Economy settings for the printer first. If the printer is in normal mode, then I advise having an HP technician look at it.
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