P0404 Code in 1999 Chevrolet Silverado 5.3L: Diagnosis and Fixes After EGR Valve Replacement

Model: Silverado Fault Code: P0404 Posted: 2012-04-15 19:02

New to the forum — hoping this is the right place to post. I own a 1999 Chevrolet Silverado with a 5.3L V8 engine, which continues to show the P0404 diagnostic trouble code along with a persistent rough idle. Despite replacing the EGR valve and thoroughly testing it for proper function, I've also inspected the EGR pipe and manifold for carbon buildup — no obstructions were found. I've replaced all ignition wires and plugs as recommended in troubleshooting guides. Recently, I received an expired inspection ticket, and my wife is pressing me to resolve this issue quickly since we're currently renting a vehicle. The P0404 code returns after clearing it, and the rough idle remains unresolved. Any insights or further diagnostic steps would be greatly appreciated.