Canon NP6621 Service Handbook page 127

Initializing Back-Up Error Codes
If the self-diagnosis mechanism has turned ON,
you can reset the machine by turning OFF and then
ON the power switch, unless the error is repre-
sented by 'E000', 'E001', 'E002', "E003', or 'E004'.
This is to prevent the user from resetting the machine
thereby damaging the fixing assembly.
If 'E000', 'E001', 'E002', "E003', or 'E004' is
indicated, remove the cause of the error and perform
the following:
1) Open the front door, and insert the door switch
actuator.
2) While holding down the service mode switch
(SW850) on the DC controller, turn the power
switch OFF/ON.
3) When the power has been removed in about 2
sec, turn ON the power switch once again.B.
Self Diagnosis on the Cassette Feeding Unit
SELF DIAGNOSIS
8–3