Never mind. There's a Macintosh version of the needed utility.

I got the idea that there was no Mac version from some of the stuff I read at fixyourprinter.com but either they were wrong or I read what they wrote wrongly.

I should be fixed up now. So, if you get a used HP printer and one tray won't work (typically reports out of paper when it is not) you may simply have a locked tray rather than a hardware problem (faulty paper present sensor).