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error HP
Error Code Er/04
Paper jam error — paper stuck in the printer pathway.
? What Causes This
Physical obstruction or misalignment of paper feed mechanisms.
✓ How to Fix It
Open access door, remove and straighten obstructed paper pieces. Unjam internal rollers if required.
Source: page 2 of the official manual
From the manual
This error code appears in:
HP Laserjet Mfp M139e M142e Printer