I get a 59.4 error trying to print with the 2840

mayby cardridges are empty

59 errors indicate one of the motors have failed. It could be a bad motor but also a bad cartridge will overload a motor and cause the error as well. If you have spare cartridges, try changing them one at a time and restart between changes to see if the error goes away. Also if the error only happens when you start to print it might be the fuser motor.

Fuser Error means that the device has experienced an internal hardware error.
1.Turn the device off and disconnect the power cord
2.Remove the fuser and then wait at least 20 minutes
3.Rotate the locks on the fuser to the unlocked position,grab the fuser handle and pull the fuser assembly from the device.
4.Using both hands, slow and steadily remove any paper or print media from the device and carefully re-insert the fuser and secure the locks.
CAUTION: The Parts on the internal surface of the fuser are very hot, so to avoid burning yourself,please do not touch those parts.
Reconnect the power cable, turn on your device and wait for it to initialize. If the error persist, Change the fuser assembly or contact HP Customer care. This instructions can also be found n your user manual. i hopes this helps.
Thanks

Try HP Support

HP Color LaserJet 2840 All in One Printer Software and Drivers

The fuser is the heating unit near the paper exit, and that error means the lamp is burned out, the overtemp sensor is out, or the thermistor in the fuser is bad. About all you can do is replace the fuser. www.precisionroller.com will have a new fuser for you.

You need to replace the fuser, a consumable item.

HP error message 50.X is usually a fuser error. replace fuser

I just reviewed both of the above fuser issues (50.2 & 50.3). Both are highly likely (even if recently replaced) fuser damaged. I have included below a link to a free HP service manual for your machines. Start on page 309 using a volt/ohms meter to test the fuser. To address the "Guest" inquiry regarding the HVPS, there is a separate HVPS however please be aware that it is not part of the fusing circuit in this machine. If anything it would be the LVPS (Low Voltage Power Supply) however very remote. Odds are maybe 1 in 200 on your best day. I'd put 10 fusers in the machine before I bought 1 LVPS. This machine is known to eat fusers. The HVPS and LVPS at this time are just about bulletproof. PLEASE TRY TESTING THE FUSER AGAIN USING THE MANUAL PAGE 309 TO 311. If your really want the power supplies though, they are on the part number list at the end of the manual. I just looked it up and we have repaired 74 of machines to date with not one PS ever used. 59 of the 74 have been for fuser issues.

Here's the manual link:
http://www.feedroller.com/Store/Service%20Manual/Color%20LaserJet%202820,%202830,%202840%20Parts,%20Service%20Manual.pdf

Hope that helps you both. :)

For any fuser error, the first thing that should be done is replacing the fuser. If the error still shows up after wards, it could be the LVPS (Low voltage power supply) or a board.

Your fuser lamp is not coming on. Reseat the fuser. If that does not help, your fuser will need to be replaced.