This is a very common problem among people who send faxes. Usually this is a relatively simple fix if the faxes are being made from the automatic document feeder. When the faxes go through the automatic document feeder, it goes over the device that inputs the faxes on the paper into the fax machine that will then be faxed out to whomever you are sending it to. There is a piece of glass which is narrow, usually only about a inch wide or so, called the, “slit glass.” If anything gets on this, “slit glass,” say for instance, “white out,” you will get a vertical black streak down your faxes. This is pretty easy to repair, all you have to do is open the document feeder door and find that little slit glass and look closely at it and see if you can find any white out or smears or anything on that piece of glass. The best way to clean it is with a moist cotton cloth and just wipe it clean. If you wanted to, you could use Windex or something close to it and clean it that way. Just make sure that when you wipe it clean and dry it, that there are no streaks when the job is done. If after you have done all that and you are still getting a black vertical streak, there may be another problem with your machine and you Need to think about placing a service call.