Have a Xerox M20i. Copy comes out light. Toner should be OK. Prints fine. Scans fine.

its probably set to a low DPI.......change your dpi to like 600 when scanning

You may not have enought toner,or the scaner needs to be clean up!Also the laser maybe need cleaning!Other thing maybe a primary charging wire witch is up on the drum!Check this thing if u can !

I would first of all Go to the Xerox.com site and go to Drivers and downloads for the M20i and install the newest firmware. I believe the current version is 3.4.
I believe your problem would be resolved in loading the newest machine firmware.

You will need to do a couple of things, 1 remove, disassemble the scanner and thoroughly clean it, and while you have the scanner out, remove and clean the laser.
The toner cartridge may also be defective. that metering blade in that cartridge fails and causes light images also.
If you need instructions on how to remove and clean the scanner and laser let me know, I will type them out to you.
Not hard, just time consuming.
If you print out a report how does that look? If prints look fine and just the copies are light, then the mirrors in the scanner head a re probably fogged up. All the plastic in that scanner causes that same film on your scanner optics as you develop on your car windshield. Same process.

Prints and Copies are lighter? You have a dirty laser.
You said Prints were fine in other posts.
Turn the machine off, remove the 2 screws holding in the left cover and pop it off. You can do one of two things. Either take a stubby screwdriver and feel around the laser for the 3 screws that hold it in and remove them. or once again remove the scanner and remove the laser from the top. There are 5 or 6 clips on the scanner , loosen them and pop the laser into half. Clean the window, all the mirrors and you will see a polygon mirror assembly on a motor. Clean each surface of the polygon mirror with a soft cloth and some cleaner, being very sure to get them spotless.
In your earlier posts you said prints were fine. That they are also light changes everything.
You could also have a bad power supply.

Since you cleand the scanner and scans are fine, the other item I seen cause that problem is the power supply.

yes directly

If the issue is only when you are copying, the scanner needs to be cleaned.
If the defect is on both copies and prints, and fax if you use it for that, then your problem is either the drum or toner cartridge or the laser is dirty.
I am not sure when you say you changed the print cartridge if you mean you just changed the toner cartridge or the drum cartridge.

someone has gone into diagnostics and enabled Status Print Engine
Fix
1. press menu/exit key
2. enter # 1931 in that order
3. engine test(diagnostic) should be displayed
4. use arrow keys to scroll to, then select engine test(status print)
5. press enter
6. press the "2" key on numeric keypad to disable (status print)
7. machine will revert to Ready to Copy
8. power off and then back on the M20
9. Your done.