| Error Message | Cause | Fix | |---------------|-------|-----| | "Driver not signed" | Windows 10+ requires signature enforcement | Restart PC → Disable driver signature enforcement (Shift + Restart → Troubleshoot → Advanced → Startup Settings → Disable driver signature). | | "No HP DeskJet 450 found" | USB cable or port issue | Use a USB 2.0 port (not USB 3.0/blue ports). Also try a different cable – the 450 is picky about cable length (< 3ft recommended). | | "File missing: hpcpu450.dll" | Extracted files corrupted | Download fresh copy. Extract manually using 7-Zip, then copy the DLL from the Drivers folder to C:\Windows\System32 . | | Printer prints gibberish | Wrong PCL version selected | In Printer Properties → Advanced → Driver → Change to "HP DeskJet 450 PCL 3" (not "HP DeskJet 450 Class Driver"). | | Blue light blinks, won't print | Bluetooth pairing lost on 450ci | Reset printer: Hold Power + Cancel button for 10 seconds. Re-pair from scratch. |