Check for vacuum leaks. I dont think your relays will act up with EGR problem. Check your fuses and electrical wires.
P0400 could be:
Plugged EGR passage which restricts the flow of exhaust gasses
Faulty EGR solenoid
Faulty EGR solenoid wiring/harness
Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
Faulty EGR valve
hope this helps
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