Half Tests

On my first blog, I discussed image defects caused by Imaging Units. If you are sure or at least suspect the IU is not at fault and don’t have another IU to check it.  Performing a half test is the way to find out.

When a laser printer creates a print, the IU first creates an image which is then transferred to the paper and then the paper moves on to the fuser, whose purpose is to melt the toner on the paper so it won’t rub off. That is the device that is hot enough to burn you when the covers are open. In performing a half test, we stop the paper before it reaches the fuser. If the defect is on the paper, then it must have been created by the IU, the transfer process or by a dirty roller somewhere in the paper path. Transfer process problems are very rare but dirty rollers are somewhat more prevalent. Most defects here are caused by the IU.  If the defect is not there, then the fuser is the likely culprit.  Common fuser defects are along the edges in HP and Canon products or in the center of most others. Center defects usually repeat about every 3 to 4.5”


If possible, use the MP, MPT, multipurpose or by-pass tray(this tray usually folds down and is used to feed unusual paper sizes, envelopes, labels and transparencies. It may simply be a slot for a single sheet of paper.). Put a piece of paper in the tray and start a print job, using the machine menu if possible. Wait for the paper to feed about ¾ of the way in and open the cover that you open to replace the toner cartridge. This should immediately stop the print process: if it doesn’t, find the door that does. Open the cover and remove the IU/toner cartridge. You will see the paper in the machine with an image on it. If possible, try to locate the defect before removing the paper because the image can rub off. Note where the defect was on the original sample, making sure you have feed enough of the paper that the defect can be seen on the paper in the machine. This may take a few times to get it right. On HP machines, you don’t have to re-enter the job to repeat the print: it will keep repeating the job until it gets through the machine.

If you can’t find any defect this way, it is most likely your fuser. Many modern machines allow the user to remove the fuser so you can inspect it. Look carefully at both the upper and lower rollers for marks or off color coatings. Be sure to rotate the rollers at least 360 degrees.

If you can’t remove the fuser, you will have to call for service.


There are about 5 different ways that color printers work and often are not easy to half test. If you have a printer that uses in-line cartridges, you may be in luck. If you have a carousel printer, you are probably out of luck.

Many color lasers print the image on a transfer belt or transfer drum first, which adds yet another possible source of defects. This can be very hard to half test because the image can be any where on a 20 to 30” belt. On machines where the paper is stuck to the transfer belt and then printed directly on the paper, half tests will probably work. HP even has a way to program stops on some of  its printers. This can be found in the diagnostics sub-menu if it has it. I haven’t had much luck with these, but it might work for you!

Next Blog: Paper jams!

If you need additional assistance, you can always call or email me.

John Scudder