I just got my car back from the Nissan dealership where I had my EGR system cleaned. The first time driving it after they cleaned it...the check engine light started flashing and it was riding really rough. I took it back saying it was something they did. They came back and said it was a bad fuel injector to cylinder 1. But that these system are unrealted. This seems like an awful coincidence to me...does anyone know how I can prove it was their fault?

 

They could possibly have disturbed the electrical connection to the No. 1 injector IF they removed the upper intake manifold. Some people do this to positively clean out the entrance passage for the EGR tube, as well as all the other carbon crud that accumulates inside over the years. Or perhaps the No. 1 coil pack connector is not fully seated.

But if not then yes, it may be just a coincidence. Either way, get it fixed ASAP. Flashing CEL's are never good.