I have two HP LaserJet 4P printers, both in decent condition, and both exhibiting the same exact problem - the printer passes all self tests and everything works perfectly EXCEPT printer output pages are completely blank or have only the slightest sheen of gray toner background.
[on first read] I think you have a high-voltage problem. High voltage is needed to charge the drum, to accept toner where the laser scanner would be charging the drum. Electrostatic voltage is the "charge" that picks up toner. The toner is mechanically transferred to the paper, the fuser fuses the toner onto the paper. Here's my reasoning which suggests diagnostics. Caution: proceed at your own risk, I am not responsible for loss injury or damages. If the output is blank, then toner is not being deposited on the drum, and/or the drum is not transferring toner to the paper. If you stop the printer mid-cycle by opening the access to the toner cart, you might be able to pull the toner cart and examine the drum visually, see if there's toner on it, in the pattern of whatever you are printing. You might have to manually rotate the (toner cart) drum, rotate it in the direction it normally turns. My guess however, is that toner is NOT getting deposited on the drum. If it were and was not transferred, then the drum would get full of toner and eventually it would dump it on the paper. So my guess is, you won't see a toner pattern. Of course, make sure your fuser is hot. But if it wasn't, the printer would issue a fuser failure. The lack of printer error codes, suggests something that is not monitored by HP printing diagostics (not reported as an error).
The service manual suggests final step would be replacing the high voltage board ... (possibly age related?) failure.
I saw that on second reading. Makes sense to me. Also another person posted the same conclusion, read as I was about to post this reply. age related? Maybe. Maybe, the humidity at your printer is so high, it shorts out the high-voltage "corona wire" which in most ancient HP printers the means by which the high voltage charge is put onto the drum. Maybe there's a mechanical fault that keeps high voltage from contacting the coronal wire. Maybe the wire is shorted to ground. Why both printers would have the same error might be apparent when you find the error. I suggest you stare hard at the 4P service manuals, stare hard at the printer mechs, track down the path of the high-voltage to the drum from the high voltage board. I do not know how lethal the high-voltage is, I suggest you determine that from HP documentation. I suggest you Web search for repairs on HP 4P printers around "high voltage" issues or other keywords like corona wire or whatever words HP uses. Someone will have discovered means to measure the high voltage. Don't just put your DVM on the line and expect the meter to magically tell you "1500 volts" or whatever it is - it's very LIKELY your DVM won't expect very high voltage and you'll blow it up! Don't blow up your meter. ----- Looking on eBay for HP LJ 4P thingies, I saw a scanner board for sale. I suspect if your scanner failed you'd get an error message. (the scanner includes a rotating mirror to scan the drum.) But listen for the scanner motor spinning up. See if you actually have an IR laser signal getting to the toner cart. - You know, if some piece of paper jammed in around the laser scanner or laser path, it might *block the laser* from shining on the toner drum. And you might have the same sort of jam on *both* printers. See if you can figure out how to confirm the laser is actually getting to the drum. You can't see the IR laser of course, but any *digital camera* is likely to be able to see it. And of course any smartphone camera. Use them in real time (camera viewfinder, phone screen) to observe IR light. I do that all the time when checking (ancient) TV cable IR remotes for operation/live batteries. Regards Herb Johnson -- Herbert R. Johnson, New Jersey USA https://www.retrotechnology.com OR .net preserve, recover, restore 1970's computing email: hjohnson AT retrotechnology DOT com or try later herbjohnson AT comcast DOT net