here is an excerpt from the lexmark technical tips library it could be usefull
Bulletin Summary: 994 Density Sensor Error
Document Date: Feb 03 2004
Operating System: Printer Based
The 994 Density Sensor Error can be caused my many different things. The basis of the Error comes from the printer not being able to do the auto calibration and the auto registration when the printer is turned on. Toner Patches are written to the belt on ever Power Cycle of the printer or when ever a door or cover is opened on the printer when the Auto Color Adjust is turned on and the Auto Registration is turned on. If too much of one color is placed on the belt or there is an absence of a color or all colors the printer will call the 994 Error. This error will also be called if the activation arm for the density sensor is not pushed in. Another thing that can cause the 994 Error is if the sensor for the under the belt for the BUD Clutch positioning is the incorrect part.
Things to Try with Customer
When the printer is powered on in Diagnostics the printer does not perform the Auto Calibration of Registration functions as would normally occur on POR. Turn the printer off and POR in Diagnostic Mode. From here print the Quality Pages under the Print Tests. These quality pages can give insight as to what is causing the 994 error.
What to Look for on the Print Quality Pages
First Look at the First Page on the Print Quality Pages. The first page tells mean things about the printer. There is a section that gives the Page Count on the printer, the Last 12 Errors, the Serial Number, Engine and Rip Code Levels. There is also a section on the first page that tells the last time the printer did the Auto Registration and Calibration. If the printer does not have 703.007 RIP on a C910 or IBM 1228 Printer this may not be displayed. Since there is only one Version of RIP on the C912 and IBM 1357's 720.013 this code will display the last time the printer was did a calibration and registration.
Second - Go through all the Quality pages are any of the color planes missing completely?
If only one color plane is missing it's possible that LED array is unplugged.
Is the top half or bottom half of one or more color planes is missing completely?
If yes then that one LED or Multiple LED's May have one Cable Unplugged.
Are all the Color pages white but the black pages printed ok?
If all the colors are missing and only black appears most likely the sensor under the Belt for the BUD Clutch position sensing is the incorrect part. This would only be the cause if that sensor was just replaced during fixing a 994 Error. Another check to do if this is the case is to look at the belt while it's in the printer. Open the Top Cover of the printer and check the right and left sides of the belt. The Belt should be raised on the right side and be equal height as the left side. The pictures below show the Belt in the Up and Down Position. If the belt is down when the printer tries to do the calibration no color will get transferred to the belt. This will cause the 994 Error. This also is why no colors would be appearing in all the planes but black is fine.
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