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· SilveradoSierra Guru
2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
3,144 Posts
If the cat's are plugged, that will cause problems, but, like above, diagnose instead of parts cannon.

Check things like:
-fuel pressure at the rail, should be 55-62 psi, both at idle and when the engine is sped up to say, 3k rpm
-all the exhaust manifold bolts are present (they are known to break off, and can cause exhaust leaks when they do, screwing up o2 sensor readings, which then screw up how the engine is run)
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