When should I replace the toner cartridge? I found that red light is flashing( the lower light, totally there are 2 lights: upper one is green now). I don't understand what does it mean.

I take it you are refilling the toner cartridge...

Switch the printer off.

Press and hold the green button on your printer and turn the printer on. You will see that the lights flash for a moment and then turn off. After a few seconds, the attention light will turn on followed by ready light and the green light. Keep pressing the green button. In a few seconds, the attention light and the ready light will turn off.

Push the toner rotate button while you continue pressing the green button. As you briefly push the toner button, you will see the light flashing. Release both the toner button and the green button.

Repeatedly push the red button (cancel button). Now all the working lights on the printer will turn off.

Open and close the lid of HP printer once. The light will start blinking.

Press the green button. Your HP toner cartridge chip will be reset.

I guess it FixYa didn't accept my (very long, but totally correct) solution that I took the time to type. Because I refused to "sign in".
Here's the short version. Load Paper. The same thing happened to me. I replaced the toner cartridge with a new one (because of a FLASHING red light for several weeks) after the red light went to STEADY red. But it still would not print. The paper was "out". When I'd refilled the paper-tray, it printed fine. I had just ASSUMED that the steady-red light meant toner was finally OUT. WRONG! That steady-red light had nothing-to-do with toner; it meant "you're out-of-paper". Don't get confused like I did. Refill the paper-tray. Then take that expensive NEW toner cartridge OUT, and put the OLD toner cartridge back in. And keep using the "dregs" of the toner out of the old cartridge for several more weeks. And DON'T get confused by the paper-is-out condition making that red light STEADY.

The steady lower light indicates the unit needs a new toner cartridge, MLT-D108S. Did you previously see this light blinking or blinking rapidly? (Those indicate the toner being low.) Replace the cartridge and check for any other error lights. Turn off the machine and let it cool. Then open the front cover and pull the old toner cartridge. Open the package with the new cartridge, remove the packing tape and paper that protects the cartridge and gently shake it (avoid touching the blue or green parts of the OPC drum). Press the cartridge into the slot until it is fully seated. Close the front cover.

The green/red blinking power light is not one I'm seen before. The printer indicates a non-toner failure (open cover, no paper, LSU failure, etc.) with a steady red light.

There are actually some certain meaning on the red light and green light showing different information to user. The summaries are:
1) Green On, Red Off = Printer Ready.
2) Green Fast-Flashing, Red Fast-Flashing = Printer warm up.
3) Green Fast-Flashing, Red Off = Printer off-line.
4) Green Slow-Flashing, Red Off = Processing Data.
5) Green Off, Red On = Cover Opened, Toner Empty or Developer Unit / Drum unit missing.
6) Green Off, Red Fast-Flashing = Paper Jam or waiting for jam reset.
7) Green Off, Red Slow-Flashing = Paper Empty or Paper tray is not inserted.
8) Green Off, Red Off = Printer power is off.
9) Green Very-Slow-Flashing, Red Off = Printer in sleep mode.
10) Green (any condition above), Red Slow-Flashing = Toner low or paper empty.

So what is your printer condition? Better know what is going on your printer so can solve the problem.

dear sir

1. it is problem paper miss feed
2. over temparature

clean thermostator.thermostator over dry toner
or cartridge assembling problem

This answer may be a bit late but if this flashing of the red and green light is accompanied by a sound of a motor starting and stopping over and over, it is NOT toner, but a defective capacitor in the power supply. Here is a photo of the part that is creating the problem. I am not responsible for any harm done to yourself so please make sure you know what you are doing.

Red Error Light Stays On The following conditions can cause the On Line LED to glow solid red:

• Printer paper is jammed inside your printer.
• A cover is not be closed properly.
• The toner cartridge is not inserted properly and needs to be reinstalled.
• The toner cartridge is running low.
• There is no paper in the paper tray.
• The paper inside the paper tray is not be installed correctly.

Red Error Light Blinks If your printer's On Line LED is blinking red, your printer's toner is exhausted and needs to be replaced. On Line and Error Light Blink Simultaneously
If your printer's On Line and Toner Save LEDs blink simultaneously, your printer requires service.

You can go to samsung online service request http://www.samsung.com/us/support/repaironline/supportOnlineServiceRequest.do

I found this and it worked great.

1. Power Unit off

2. Open front cover

3. Press and hold GO and turn power on

4. When all 3 lights lit release and press GO twice

5. All lights turn solid

6. With all solid, press GO 5 times

7. Ready light GOes red

8. Close door and you're all set

I had this problem and couldn't figure out until I opened the front, pulled out the toner cartridge and found a paper jam.  Removed it and now I have the green light.

The green flashing light lets you know that the machine is recieving data. This is completely normal, however, the red (Attention) light can mean several things. There can be a door open, missing toner cartridge, out of paper, or paper jam. In this cause it is probably the toner cartridge. Typically on these machines there is an Orange or Green tab on the side of the toner cartridge that needs to be pulled to fully engage the cartridge. You may want to make sure this has been pulled and the cartridge has been properly seated.

Hope this helps...let me know if you need any further assistance

V