I wish I knew more about the EGR. But if you wanted to avoid the dealer at all cost, you might try replacing some of the major EGR components. They are not that expensive and they look easy to replace, such as the 1) EGR valve and 14) Solenoid.
But I really don't have a clue.
Here is info from the Service Manual for code PO404
The EGR flow or valve movement is not what is expected.
(F856) 5-VOLT SUPPLY CIRCUIT
(K35) EGR SOLENOID CONTROL CIRCUIT
(K900) SENSOR GROUND CIRCUIT
EGR SOLENOID ASSEMBLY
Direct PARTS connection
1) Egr valve 2) Egr valve Gasket 3) Egr tube
4) Egr tube Seal 5) Egr tube Gasket 6) Leak detect pump
7) Hose To leak detect pump 8) Filter 9) Filter hose
10) Vent hose 11) Vapor canister 12) Vapor canister Grommet
13) Vapor canister Bracket 14) Solenoid 15) Solenoid Bracket
16) Vacuum harness 17) Oxygen sensor Before catalyst 18) After catalyst After catalyst Right