Basically high Nox is to much heat in the cylinders. Egr system introduces exhaust back into the intake to “cool” the cylinder temperature. Carbon deposits are another factor that can glow in the cylinder and cause problems. To much timing is another factor. Make sure the egr diaphragm is actually moving and that the passages are not restricted. Keep timing as close to 0 as possible for testing. Try to de carbonize the cylinders.