Printing Errors or Lights & Stuck Print Jobs

I'm getting a "Carriage Blocked" error on my faithful old c5280 printer. The carriage moves freely, there is no paper jammed, and the printer is meticulously clean (as far as I can see) but this error message is persistent.

I found and followed all the HP-posted service procedures on the subject, and even followed the Semi/Full Reset procedure posted below (for reference). Everything looks great on this full reset until I get:

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! Carriage Blocked

Raise cover and remove cardboard or tape that is preventing the carriage from moving.

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Again, there is no obstruction and the carriage is moving freely. If I open the cover while it's "setting up," the carriage is moving around on its own with no problem.

Could there be a hardware sensor that is obstructed or damaged that I could free up? I know this is an old printer, but it has been a faithful friend for years.

Semi/Full Reset procedure I found archived from this forum:

do a semi full reset

combo would be:

•"print photo" + "red eye removal" buttons  pressed together

•printer will not ask you to enter the combo

•release the buttons you pressed then

•press "red eye", "print photo", "red eye" in sequence

•scroll down or right  until you find resets menu

•press ok

•scroll down or right until you find semi-full reset

•press ok

•your printer should turn off, wait for 3 minutes before turning the printer back on