SES light suddenly appeared a few days ago. I took my Mustang to AutoZone. Code P0402 (Excessive EGR at idle)was the only one there. I removed the EGR, but it looked ok....No carbon and it didn't appear to be stuck open.

After a lot of Internet research in several threads on this board, it appears the DPFE sensor fails much more often than an EGR valve. After replacing the $34.99 sensor (from NAPA), it appears the problem has been resolved.

Thanks everyone. You all do a great job!!