The HP Color LaserJet (CLJ) 2550 / 2550L / 2550LN uses the toner cartridges Q3960A, Q3961A, Q3962A and Q3963A. The printer stops printing after some defined number of pages although the toner is not really empty. This is really annoying. The HP toner is very expensive and the toner cartridges which are supposed to last for 4000 (5000 black) pages only last for about 2000 pages in practice (at least for me).
To make use of the remaining toner there is a workaround. This requires tampering with the toner cartridges and you will loose the warranty for your device. You may also damage your device, so do not do the following if you do not know what you are doing!
Disclaimer: Do not follow any of the procedures below! They are unsafe. You have been warned!
When the LED is flashing for some toner cartridge: Take it out and swap the chip with the chip of some other toner cartridge of a different color. This can be from an empty toner cartridge. It's a good idea to mark the chip with the original color as it cannot be used for this color in the future. The printer will now claim that there is a non-HP toner in the printer, but it will print as normal. The LED of that cartridge will be off. When I did this for the black cartridge I always had to switch the printer off after each print job as it would recognize a fault on black. I also had to open and close the paper compartment every 50 pages or so. Btw, this also helps when one of the toners gets empty (blinking) during a long print job: When the printer stops open and close the paper compartment, then it will continue to print. You need to take care of the image quality yourself now of course. Destroying the chip with 12V did not work for me for the toner cartridges.
For the imaging drum swapping the chip did not work for me. But destroying the chip by applying a Voltage of 12-14V for 10 seconds to the two pads worked for me. The drum LED is now also off and recognized as non-HP. I also needed to get the old toner out of the imaging drum: It can be opened by removing the screws from one side and removing the white gears and the cover of the old toner compartment. This was very tricky to do and I had to almost break the platic parts. I took out a long threaded platic part, The old toner causes a big mess! Do this outside! Wear old clothes! Use an old or industry vaccum cleaner to remove all of the old toner. Don't use you home vaccum cleaner for this, the toner goes thorugh all the filters. Do this outside! This may seriously damage and break your imaging drum. But it does not work anyway if the drum LED is blinking. I made the mistake of touching the white platic sheet, so I had my fingerprints on the first 10 pages I printed. However, they fade out by themselves. Avoid touching the imaging drum itself by any means.
The original ideas came from this german forum post: here.
My imaging drum still works perfectly (but only after I removed the old toner from it). And the toner cartridges can print many pages after they initially stopped printing!
BTW, all the hold keys down while switching on did not work for me at all!
Second imaging drum was depleated today. The last color pages printed were absolutely flawless. The locking mechanism is annoying. I tried to burn the cjip with 12V like the previous one, I even used the burnt chip which worked on the previous drum, nothing worked. The drum LED would blink and the printer was not ready. Pressing cancel didn't help and also all the 'hold down buttuns which powering on' stuff did not work at all. No effect.
I finally (already prepared the throw the whole thing into the trash including 4 new toners) tried an old empty chip from a yellow toner. This worked! The drum LED was blinking and the attention light was on. Pressing cancel cleared both LEDs. Now the drum LED is off and the printer print again. (For the first page there is no ready light though. However this cured itself later as well, nit sure when.)
My conclusion is: Do not buy HP printers. You will get a big pile of trash for a lot of money. Looking back the years, HP2550: Expensive toner, expensive drums, lots annoyances, lots of paper jams from bottom compartment. Very slow printer. Crappy HP printer driver (always prints color pages, also for BW pages). Bad support for Mac OS X.