Faxing out the other party gets a thick black line on our fax?

It really depends on how thick the black line is. If its say 1/2" to 1" thick then a segment of the contact image sensor has failed. If its very thin then that would be just dirt either of the scanner glasses or under them. When you open the scanner cover you will find two glasses. the thinner one is for the adf and the very large is for flat bed scanning. If you get the line when using both then the CIS is faulty and needs replacing. If either one is good with no line then it could be just the ribbon cable that goes from the main board to the CIS.

Check the fusing unit and or transfer belt. Does it print OK not copy! Print Ok means internal OK then check the scanner section

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If it is just on faxes you are receiving and not on ones you send it sounds like it may be your drum. You may have a small tool in the fax to gently rub across it and clean it. check your manual to make sure. In worst case scenario you will have to get a new drum. Look on line for your model. Other wise if it's on copies and faxes you send as well it's going to be the thin piece of glass right where the pages are being feed in that needs to be cleaned

The drum is your problem

Your scanner glass is caked with ink or lint. Open front cover and clean with windex or rubbing alcohol.

Something is jammed in or covering the optics.
Check the area where the original document is placed for faxing.
Open this area and clean any liquid paper, sticky tape or staples covering the optics.
Test by making a copy. When the black line on a copy goes away the unit is fixed.
If the receiver is still getting this black line then it is probably their machine and not yours.
Send a fax to this number. 1300 368 999 It will send you back a copy of what it receives.

Look on the glass where the scanner is and sometimes a lot of White out gets stuck on the glass, clean with windex and dry good.

If it's a lazer fax, look on the rollers for toner build up around the fuser section. Clean with Alcohol.

Open the document feeder, there should be a narrow, long scan glass underneath. Clean that glass. Sometimes whiteout (liquid paper) or other foriegn matter will stick to the scan glass causing a "shadow" (line) on faxes being sent out. Hope this helps.

Open the front of the fax machine. Then pull down the the tray that has the down arrow imprinted on it. Locate the glass strip towards the back, just behind the center roller. Use a cloth and rubbing alcohol to clean the glass and that should remove the black line.

The drum cleaning blade is worn out. You must replace the Drum Unit.