Dell 1720 new cartridge and error light stays on and says low toner

First of all dirty rollers are not the cause of your problem as long as the printer is picking up the paper.
Black marks or streaks on a DELL 1700 series printer are usually caused by the drum assembly which, in this model, is separate from the toner cartridge. There is no repair, you need to replace the drum assembly and perform the drum reset procedure. In very rare cases a buildup of residual toner on the fuser can cause marks as well, but I have never seen this on a DELL printer.
To replace the drum:
1. After removing the paper tray, open the front of the printer.
2. Remove the toner cartridge by pushing down on the black release tab and pulling the toner up and out of the printer.
3. Pull the drum assembly straight out of the printer using the blue plastic handle.
4. Carefully unpack the new assembly and install in the printer. Be careful not to touch the photo-sensitive roller underneath.
5. Reinstall the toner cartridge, close the printer and insert the paper tray.
6. Perform the drum reset procedure exactly as follows:

power off and open front door hold down the continue button as you power on printer leaving front door open when error light comes on release continue and close front door. toner low, load paper, ready, and continue will all be lit. press cancel slowly 4 times or until just toner low lights up. each time you press cancel, the light sequence should change when just toner low is lit, press and hold continue button until lights cycle up and down in sequence. release continue.. toner low will be lit again turn printer off, wait 10 secs or so and then back on press continue button to print off settings page and check under cartridge information, photoconductor value is 0K (zero)

If the drum cartridge has not been replaced, it must be replaced before resetting the counter to avoid damaging the printer.

1. Power off the printer.
2. Open the front door.
3. With the front door open, hold down the Continue button while powering on the printer.
4. Keep holding continue until all lights start flashing then release the button.
5. Once only the Error light is on, close the front door.
6. At this point Continue, Ready/Data, Toner Low, and Load Paper should all be lit.

If the previous set of steps did not go as described then do not continue. Please complete the previous set of steps again.

1. Wait for the printer to make a sound as though it is about to print something. (This may take a minute.)
2. Press Cancel 4 times SLOWLY (1 second between presses) to cycle through lights until ONLY the Toner Low light is on. (It may take more than 4 presses.)
3. Press and hold Continue until all the lights cycle through and go back to Toner Low
4. Cycle power on the printer.
5. Press and release the Continue button to print out the settings page. Under Cartridge Information it should read Photoconductor: 0K

Hello

You have to check couple things in this case.

Ensure that the printer cover is closed.

1. Remove the toner cartridge.
   
2. Firmly shake it side-to-side and front-to-back several times to redistribute the toner.
   
3. Reinsert the toner cartridge, and continue printing.
   

Repeat this procedure multiple times until the print remains faded. When the print remains faded, replace the toner cartridge.

or
The imaging drum is approaching full and needs to be replaced. Order a new imaging drum.

Open the front of the printer then hold down the cancel button. When the lights flash close the front and it should Reset.

Open the front cover by pressing the button on the left side of the printer and lowering the cover.
Press the button on the base of the toner cartridge assembly.
Pull the toner cartridge up and out using the handle.
Unpack the new toner cartridge.
****Rotate the cartridge in all directions to distribute the toner****
Install the new toner cartridge by aligning the white rollers on the toner cartridge with the arrows on the tracks of the imaging drum and pushing the toner cartridge in as far as it will go. The cartridge clicks into place when correctly installed.
Close the front cover.

You have to reset the drum count on the printer after you replace the drum. Call dell tech support they will not charge you for help resetting the drum even if you are not in warrenty.

Leave the printer on. Open the front and hold down the cancel button until all the lights are flashing in sequence. Close the front while still holding the cancel with the lights flashing and then release the cancel button. This should reset it.

Thanks

You should still be able to do a printer test page (press the arrow button).

Look at the printed page(s) for the Photoconductor AND the Toner.

They show as OK when new (or very low count), and 100% when fully used.

It IS ossible that the toner cartridge has a faulty (or if a remanufactured cartridge - un replaced) chip.

Some 'after market' chips are not compatible with E250, and it IS possible that your replacement cartridge is for an E350, or E450 (which have different chips, and some of the poorer remanufacturers , and some reselers that don't do these very often don't know).

It could even be a Dell 1720 chip on the cartridge.

I make around150 Dell 1720, 20 E450, 50 E350 cartridges a month and only around 5 E250s, so this gives you an idea of the relative numbers, in the market

If both Lights are solid, it means that the top cover is still open and the toner is low. Please make sure the top cover is closed all the way. If the toner light is blinking and the errror light is solid, it means there is a toner cartridge error. Please try a new cartridge. If that solves the issue, then you should return the damaged one.

Try resetting the error lights with a drum reset. With the power on and cartridge door open, press and hold the Cancel button until all lights flash in sequence. Release the button and close the door.