!
error HP
Error Code Power LED blinks red, 3 blinks with a pause
Processor not installed.
? What Causes This
Missing CPU or improperly seated processor.
✓ How to Fix It
Insert/replace the CPU and verify seating. If issue persists, contact service.
Source: page 2 of the official manual
From the manual
This error code appears in:
HP Prodesk 600 G1 Small Form Factor PC