Customer recently bought a car to me complaining of rough running and mil illuminated. He had recently bought it and had been told that the egr valve had been replaced. Sure enough the fault stored was .EGR flow rate to high. Firstly I used a KTS 650 to do the basic setting on the valve incase this had not been done. The car ran fine so he took it away two days later it was back same problems. This time though the scan tool also named the o2sensor so I scoped it and found it wasn't switching so replaced it deleted the fault and it didn't come back the same can't be said for the egr valve though! Looked on the wiring diagram and found that it shares a wire with the crank, cam, coolant temp and map sensors. Should I scope these sensors? Could their performance be affecting the flow rate? I've checked for air leaks and nothing has been spotted yet? Am I heading in the right direction or am I way off?? Cheers guys.