If the error persists, technicians often use the Service Mode (Utility/Counter > Stop > 0 > 0 > Stop > 0 > 1) to "Initialize" the system and clear persistent error flags.

No. The hard drive stores spool data and address books. Code 9903 is unrelated to HDD. Do not waste money on HDD replacement for this error.

: Dust on IDC sensors or the toner path can trigger false jam signals. File Complexity