My MIL light keeps coming on with DTC code P0401. "Insufficient flow detected in EGR system." I've replaced the EGR valve. But about 2 weeks later, same error "P0401." Now I can't even drive around the block after resetting the MIL without it coming on again. I've inspected the EGR valve and manifold port and didn't see any deposits other than a slight black dust.

Anyone have any suggestions? Thanks.