The two black ink tanks serve different purposes and are used for different types of printing. The larger black ink tank is a pigment-based ink and is used for your document printing when plain paper is selected, you are doing single-sided printing, and your document contains black text. The smaller ink tank is a dye-based ink and is used when printing photos, performing duplex (double-sided) printing, and when a paper type other than plain paper is selected (i.e., envelopes, labels, photo paper, etc.). The smaller black ink is also used when printing certain webpages and PDF files, even if the text in those documents is only black.
When the smaller black ink tank runs out, you are not able to use the larger black ink tank in its place unless you are able to highlight the text in your doument and change it all to black text. You will also need to have the plain paper option selected as the paper type and be printing single-sided. If another paper type or duplex printing is selected, the printer will attempt to use the smaller black ink tank.
Hope this helps!
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