I am getting a low photoconductor warning. I changed them and I am still getting the warning. I turned the printer off and still I get the warning.

If you get this warning on your printer, a replacement of your photoconductor unit is needed.
Part #: E250X22G ( Photoconductor Unit )

https://parts.lexmark.com/wpo-catalog/US_en/

Once the unit is replaced, you have to reset the Photoconductor count. To do this, please visit the link below.
http://bitly.com/HZZ19o

Hope this helps!
For further assistance, please contact Lexmark Technical Support.

You get the PC life warning when the photoconductor kit is near exhaustion. You have to prepare are replacement.
Please click on the link below to get instructions on how to reset pc count. Whenever you install a new photoconductor unit, you must also reset the page count on the printer.
http://bitly.com/rgOZIm

Hi,

1. Enter the printer's Configuration Menu. To do this:
2. Turn off the printer.
3. Open the front cover
   
4. Press and hold the Continue () button.
5. With the front cover open and the Continue button pressed down, power on the printer.
6. Hold the Continue button until all the lights cycle.
7. When just the Error light (!) is displayed, close the front cover. The top four lights of the operator panel should come on, indicating that the printer is in configuration mode.
   
8. Print the Configuration Settings page. To do this:
   
   • Press and hold the Continue () button until all of the lights flash. This page lists the functions within the Configuration Menu. The functions are the non·indented items listed along the left edge of the page (along the bottom as printed from printer). They are also unshaded.
     
     Save this page for possible later use.
     
9. Reset the photoconductor page count as follows:
10. Press and release the Cancel button () to move sequentially from one menu item to another. NOTE: If you get lost, you can press and hold the Cancel button to jump to home state (top four lights on).
11. Press and release the Cancel button until the Toner Low () light is the only light left on.
12. Press and hold the Continue button until all of the lights flash.
13. Then release the button. When the Toner Low light is once again the only light left on, the photoconductor page count should be reset to zero.
    
14. Return the printer to normal mode. To do this:
    
    • Power it off for five seconds and then turn it back on.
      
15. Print a menu settings page. To do this:
    
    • When the Ready () light is on, press the Continue button once. The Ready light will begin flashing and one or two pages will print out.
      
16. Check the photoconductor reading on the menu settings page. To do this:
17. Look for a section labeled Cartridge Information and locate the word Photoconductor under that heading.
18. Confirm that the photoconductor level reading is OK or close to 100%.
19. If the photoconductor reading is low, try printing 10 - 20 pages.
20. Print the menu settings page(s) again. Confirm that the photoconductor level reading is OK or close to 100%.
21. If the photoconductor reading is still low, refer to the Configuration Settings page you printed in Step 2.
22. Confirm that Reset Photoconductor Counter is listed on the page.
    Also confirm that the light sequence for the photoconductor counter is the Toner Low light.
    • If the light sequence is correct, try repeating this entire procedure again from the beginning to reset the counter once more.
    • If a different light sequence is listed for the photoconductor counter, repeat the procedure but this time, instead of stopping at the Toner Low light (Step 3.2), press the Cancel button until the operator panel shows the light sequence that corresponds to the one listed on the Configuration Settings page. Continue with the remainder of the procedure as before to reset the counter again.
23. If the photoconductor reading is still low after the page reset procedure has been performed twice, the printer may require service.

Here is an excerpt I found from the W840 user manual. Looks like you need to change the photoconductor kit. Here is the link for the user manual: images.lexmark.com/publications/pdfs/w840/eng/ug.pdf
To ensure print quality and to avoid damage to the printer, the printer stops operating after the photoconductor kit has reached approximately 60,000 pages (Letter/A4). The message 84 Replace PC Unit is displayed when the printer reaches this point. If the Toner Alarm setting is set to Single or Continuous (default is Off), the printer automatically notifies you before the photoconductor reaches this point and displays the message 84 PC Unit Life Warning. To avoid having the printer stop operating, replace the photoconductor kit when you first receive the 84 PC Unit Life Warning message. Once the photoconductor kit has been replaced, normal printer operation resumes

When you replaced the photoconductor, did you go through the procedure to reset the photoconductor counter ?

There's no chip on the photoconductor, so you need to reset this manually


Photoconductor Reset Procedures:
Step 1: Open the front door and hold the
Cancel button until all lights begin to cycle.

Step 2: Close the front door. The warning
clears and the counter resets to zero.

Step 3: Press Continue to resume printing.

The photoconductor needs to have life left in it for printing and scanning.

The drum collects used toner for apprx. 30,000 pages. After that, the printer recommends that it be changed to prevent used toner from spilling internally into the printer's mechanisms. The whole photoconducter/drum can be replaced. The new part from Dell is apprx. $50.00 US. 
http://accessories.us.dell.com/sna/products/Printer_Accessories/productdetail.aspx?c=us&l=en&s=dhs&cs=19&sku=310-8710#Overview

You might have reset the "counter" for the photoconducter. Ever  have a water  fillter with an electronic timer? You replace the filter, then you reset the timer? Perhaps you reset the timer without actually replacing the filter. So to speak. :)
The question you should ask yourself is: would I like to spend $30 - $60 [it's cheaper for refurbished parts- Google it] replacing the drum head on a 7 year old printer, or would I rather invest that in a new printer?

1. Enter the printer's Configuration Menu. To do this:
2. Turn off the printer.
3. Open the front cover, as shown below.
   
   
   
   
   
4. Press and hold the Continue () button.
5. With the front cover open and the Continue button pressed down, power on the printer.
6. Hold the Continue button until all the lights cycle.
7. When just the Error light (!) is displayed, close the front cover. The top four lights of the operator panel should come on, indicating that the printer is in configuration mode.
   
8. Print the Configuration Settings page. To do this:
   
   • Press and hold the Continue () button until all of the lights flash. This page lists the functions within the Configuration Menu. The functions are the non·indented items listed along the left edge of the page (along the bottom as printed from printer). They are also unshaded.
     
     Save this page for possible later use.
     
9. Reset the photoconductor page count as follows:
10. Press and release the Cancel button () to move sequentially from one menu item to another. NOTE: If you get lost, you can press and hold the Cancel button to jump to home state (top four lights on).
11. Press and release the Cancel button until the Toner Low () light is the only light left on.
12. Press and hold the Continue button until all of the lights flash.
13. Then release the button. When the Toner Low light is once again the only light left on, the photoconductor page count should be reset to zero.
    
14. Return the printer to normal mode. To do this:
    
    • Power it off for five seconds and then turn it back on.
      
15. Print a menu settings page. To do this:
    
    • When the Ready () light is on, press the Continue button once. The Ready light will begin flashing and one or two pages will print out.
      
16. Check the photoconductor reading on the menu settings page. To do this:
17. Look for a section labeled Cartridge Information and locate the word Photoconductor under that heading.
18. Confirm that the photoconductor level reading is OK or close to 100%.
19. If the photoconductor reading is low, try printing 10 - 20 pages.
20. Print the menu settings page(s) again. Confirm that the photoconductor level reading is OK or close to 100%.
21. If the photoconductor reading is still low, refer to the Configuration Settings page you printed in Step 2.
22. Confirm that Reset Photoconductor Counter is listed on the page.
    Also confirm that the light sequence for the photoconductor counter is the Toner Low light.
    • If the light sequence is correct, try repeating this entire procedure again from the beginning to reset the counter once more.
    • If a different light sequence is listed for the photoconductor counter, repeat the procedure but this time, instead of stopping at the Toner Low light (Step 3.2), press the Cancel button until the operator panel shows the light sequence that corresponds to the one listed on the Configuration Settings page. Continue with the remainder of the procedure as before to reset the counter again.
23. If the photoconductor reading is still low after the page reset procedure has been performed twice, the printer may require service.

According to Lexmark E450DN service manual;
1. Turn off the printer.
2. Press and hold Checkmark and Right Arrow buttons.
3. Turn on the printer.
4. Release the buttons when 'Performing Self Test' displays.
The message CONFIG MENU displays on top line of operator panel.
5. Scroll down and select 'Reset PC Cnt.'

To reset the photoconductor page count:

1. Turn off the printer.
   
2. While the printer is off, press and hold the Select () and Return () buttons on the operator panel.
   
   
   
   
3. While keeping both buttons held down, turn the printer back on. Keep holding down both buttons until the printer screen displays Performing Self Test or CONFIG MENU.
   
4. Release the buttons when you see either of these messages.
   
5. If Reset PC Cnt does not then appear on the screen, press the left or right menu button until it does.
   
   
   
6. Press the Select button once. The message Resetting PC Cnt Value should briefly appear on the screen, indicating that the photoconductor page count has been reset.
   
7. When the display shows CONFIG MENU again, power off the printer.
   
8. Wait five or more seconds and then power the printer back on. NOTE: Do not hold any buttons down when powering on the printer during this step.
   
   The photoconductor page count should now be reset. At this point, it is recommended that you print a menu settings page to confirm this. Do the following:
   
1. With the printer at the Ready status, press the left or right Menu button until UTILITIES MENU appears on the screen.
   
2. With UTILITIES MENU on the screen, press the Select button once. A new menu should appear.
   
3. If Print Menus is not displayed on the second line of the screen, press the left or right menu button until Print Menus appears.
   
4. Press the Select button once. The message Printing Menu Settings should appear on the screen and the printer will print out one to three pages of printer settings.
   
5. Look for a heading labeled Cartridge Information on one of the menu settings pages.
   
6. Under Cartridge Information, look for the word Photoconductor. Confirm that the level of the photoconductor says OK or is close to 100%.
   
7. If the photoconductor is still reading low, try printing 10 - 20 more pages.
   
8. Print the menu settings page(s) once more. Confirm that the photoconductor level is now at OK or close to 100%.
   
9. If the photoconductor continues to give a low reading, repeat this entire procedure again from the beginning to reset the counter once more.
   
10. If the photoconductor reading is still low after two attempts to reset the counter, the printer may require service.