I have hp lj m401 error 78.How to fix this since always showing in the panel of the printer http://www.fixsumo.com/

did you read the HP page, on your M401 page.
and read the trouble shooting guide there,
all error codes are there, to read 24./7.
or ask here and they will point to the page. for you.
https://h30434.www3.hp.com/

The correct solution is to replace the scanner with the correct M401 model not one from an older HP. The scan speeds could be different, the motors could be different which would make the formatter think that the motor was turning too slow.

See this... http://h30499.www3.hp.com/t5/Printers-LaserJet/Hp-Laserjet-500-M551-error-code-98-01-01/td-p/5983333#.U6raidIg__8

http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=en&cc=us&docIndexId=64179&taskId=135&prodTypeId=18972&prodSeriesId=412144

Try looking at the Software Technical Interface.This looks a job for the HP experts.

i have many times working on this printer and i always experience this problem...i think this problem is disalignment of the main gear...check the gear if its on the alignment and its work for me and the problem never comes back

www.feedroller.com/Store/.../HP-LJ-M1005-MFP-Manual.pdf
service manual above. Fix it with this engineers manual.
All error codes and fixes at above manual download and read.

Hi,
you need an maintenance kit 4100

ACC C8057A Maintenance Kit, Fuser, 110V, LJ 4100 LJ 4100 Series ACC C8057-67901 Maintenance Kit, Fuser, 110V, LJ 4100 LJ 4100 Series ACC C8058A Maintenance Kit, Fuser, 220V, LJ 4100 LJ 4100 Series
Selected Item RG5-5069-000 See RG5-5069-090 Not Available 0
Alternate Item(s) RG5-5069-090 Fuser Film ***'y, 220V, LJ 4100 180.50 2 http://www2.printerworks.com/cgi-win/tpwall.exe/find?RG5-5069-000

HP LJ 4100 Fuserkit RG5-5069 HP

Try the following steps to reinitialize the HDD:

1 Turn the printer off.
2 Turn the printer on.
3 Press and hold PAUSE/RESUME (LJ 9500) or START (LJ 9500mfp) on the control panel during the
memory count.
4 Hold PAUSE/RESUME (LJ 9500) or START (LJ 9500mfp) down until all three error lights illuminate
steadily.
5 Press (the 5 button on the MFP) until INITIALIZE DISKS appears on the display panel.
6 Press (the 6 button on the MFP).
7 The printer begins initializing the disk (this might take a moment). The control panel displays the
HP event logo and eventually displays READY.