I'm going to venture a guess and say that you are the end-user and not a technician. There are some things you can check, but trust me when I say that something needs to be done by a trained tech. Even us trained techs screw stuff up sometimes.
There are a few things you can check/try.
Are you using OEM (Lexmark brand) or 3rd party cartridges?
If it's 3 party, consider trying a different brand. Maybe defective cartridges
Are you using a combination of OEM and 3rd party at the same time?
Use all OEM or all 3rd party, do not mix and match.
I've never had a C912 do this (I've only seen one of these), but I've had an HP 4600 do it. With the 4600, every time you print from tray 1 (multi-purpose tray not the cassette) with the paper set to ANY size, the printer automatically releases toner and performs a cleaning after every job. It also happens every 90 pages or after each calibration. When customers use tray one like this and prints short edge feed, extra toner gets deposited on the transfer belt. Also happens if the customer prints edge to edge and the print is off the page. Extra cleanings deposits more toner in the waste toner hopper. When the hopper is full, a clicking sound occurs, extra toner dumps in the machine, and cleaning blade streaks occur (causes streaking in the printouts).
1.) Change tray 1 size to letter when printing letter (from control panel & from the software. remember software overrides whatever the control panel says).
2.) If you are in a habit of frequently calibrating, try reducing the number of times you do this (only as necessary)
3.) Have your IT person upgrade the firmware and download the latest drivers. If there is a problem with this, Lexmark may have made a fix in their firmware to correct the problem.
4.) It could also be that your waste toner hopper is full. It doesn't appear to be something a customer could do, so you might want to get a trained tech to check on that for you. It seems to be near or a part of the transfer belt. Do not turn the transfer belt upside down as the waste toner will contaminate it and you'll have a big mess.
After trying that if you still have a problem, call a trained technician. It could be one of the following, but you'll need training to know what to do, how to do, and how to be safe. Calling a tech would be your best decision.
Density sensor actuator may be stuck, sensor may need cleaning, or be defective. Density controls how dark the image is. More toner is used the darker it is. If the sensor is dirty or defective, it can't read density, so it keeps darkening everything until it can. Usually error codes would be present, but it may not display it.
Defective controller board - not cheap
I hope this helps you. Please let us know if the problem gets resolved and what fixes it. This helps us be better techs too.
Lexmark C912 Help 
I have a lexmark C912 that has been a pleasure to operate, until now...first there was a rattle/clicking noise. I noticed that magenta toner was spilling onto the documents. Looked inside and magenta toner was spilled inside the unit, on transfer belt, on the sides etc...changed toner cartridge and developer. Fixed the problem, but after about 300 prints, started again. What is causing this? any suggestions would be appreciated.
Linear Mode
