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Status Ready LED and Attention LED start blinking simultaneously

LED indicator pattern — both Ready and Attention LEDs are blinking together.

? What Causes This

Network settings reset in progress or network restore operation triggered.

What This Means

This indicates that pressing and holding the Wireless button has triggered a network settings restore. Wait for the operation to complete.

Source: page 9 of the official manual

From the manual

Manual page 9 — HP error Ready LED and Attention LED start blinking simultaneously

This error code appears in:

HP Color Laserjet Pro M255 M256 Printer