your printer's scenario was exactly like my device. even when a new black ink toner was installed, b&w prints came out light & faded 
. another community member named nivek posted the following link which solved my device's light/faded b&w output:
just followed the 4 steps under the section called Entering Service Diagnostics, rebooted my printer and sent a few text-only test pages. by the 2nd page job, normal black levels finally appeared 
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good luck!