06 mustang gt with Roush package sport has engine check light on code shows PO 403 Can you explain code?

PO403 Is rather generic. It simply means that the circuit has failed somehow. This means that the connector on your Exhaust Gas Recircultion sensor could have simple come off. I recommend that you take this to your Ford dealership. They can pin-point the prblem. They have to fix these types of problems for free for a certain amount of time and mileage. It is a government thing. Hope this helps.