HP Deskjet 3520 Won't Print Black - Solved

The other day I noticed that my black printouts are getting fainter and fainter. So I switched to printing in colour, while I want for the replacement cartridge to arrive. I installed the new ink cartridge, and my first black printout turns out blank. Not faded or faint - the page is totally blank. I thought maybe it takes a while for the ink to go through the system, so I printed a few more sheets - still blank. Printed a few test pages with colour and grayscale. They all came out fine, except for the parts where there's supposed to be black ink.

A quick Google search tells me this is quite a common problem. Some suggestions:
 - Reseat the cartridge as it might not be installed properly.
 - Run multiple passes of "clean printhead" as it might be clogged.
 - Check the vent of the cartridge as it might be clogged. In my case, I noticed that the whole upper sticker/label on the cartridge was peeling off, so I taped it back on to provide a better seal/suction.

As part of my troubleshooting, I put back the old cartridge as it should be good for at least one or two pages. Still blank, so definitely nothing to do with the cartridge. At this point, I'm thinking the firmware/software must've decided NOT to allow black and white printing for some reason. Did a partial reset using this procedure from YouTube, then did a couple of "print status report", "print qualit test report". At one stage, I started getting black print again. Then the printer said ink is low and cartridge requires replacement. Put back the new cartridge I bought, and everything is good from then on.

MD5 Checksum

Lots of ways to generate md5 checksum for a file. Easier to do on Linux than Windows. Just run md5sum.

Normally, I would download and install software like:

• HashTab (http://implbits.com/products/hashtab/)
• HashCheck (http://code.kliu.org/hashcheck/)
• HashMyFiles (http://www.nirsoft.net/utils/hash_my_files.html)
• Summer Properties (http://www.frozenlogic.org/summerproperties.shtml), or
• File Checksum Integrity Verifier (http://download.microsoft.com/download/c/f/4/cf454ae0-a4bb-4123-8333-a1b6737712f7/Windows-KB841290-x86-ENU.exe) from Microsoft itself.

If your system is locked down with no admin rights to install new software, try the built-in CertUtil utility in Windows.
Syntax: certUtil -hashfile pathToFileToCheck [HashAlgorithm], where HashAlgorith can be MD2 MD4 MD5 SHA1 SHA256 SHA384 SHA512