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We need year, make, model and engine size.
What were the specific code numbers.
Guessing at it can just send us a long way down the wrong road.
I'll give you one common failure in Ford EGR systems but this is only a likely probability. They are notorious for a bad PFE sensor(Pressure Feedback EGR). They cost about $40 and fail frequently.
Sure, there are other possibilities. There are specific codes stored also. There are also other self tests that the vehicle can run by using a scan tool. If you want accurate answers, then we need accurate information.
We need exact codes from
Stored memory codes
You can have these tests done, record all codes and try this again.