2002 Silverado 2500HD 6.0 Vortec Gas Engine: Replaced Two New EGRs but Still Shows P0404 Code

I own a 2002 GMC Silverado 2500HD with a 6.0L Vortec gasoline engine. After the check engine light triggered a P0404 code, I replaced the EGR valve once—still no improvement. The code persisted, so I returned the first EGR and installed a second brand-new EGR valve. After clearing the code, a pending P0404 appeared during the drive cycle and eventually set on the second trip. While driving with the check engine light on, the EGR system monitor becomes ready, but the P0404 code remains active. I have already inspected, cleaned, and removed the EGR pipe, and replaced the EGR electrical connector. Despite these efforts, the issue persists. I am unable to pass a smog test with this vehicle due to the ongoing P0404 fault. Any insights or troubleshooting steps for this persistent code would be greatly appreciated.