!
error HP
Error Code Red LED on continuously (or red blinks)
The unit has encountered a severe issue leading to system stoppage or toner end-of-life.
? What Causes This
System error or near imaging unit expiration.
✓ How to Fix It
Follow display message instructions for clearing; contact service if problem persists.
Source: page 130 of the official manual
From the manual
This error code appears in:
HP Laserjet Mfp M438