Both trouble codes P0403 and P0406 indicate an "electrical" problem with your vehicle's EGR system. In very rare cases the codes are triggered due to vacuum leaks or plugged EGR ports; due to high carbon buildup. On later model vehicles, this is not common. So, first thing we'd recommend, having already replaced the ECM and the EGR valve (twice)... check the EGR wiring from the computer and back, and also all connections/connectors within; mainly the connector at the EGR valve itself. We've seen plenty of vehicles where the pins within the connector have been damaged while removal and replacement of the EGR valve. The pins are thin and easily bendable.
P0403 is more of a general electrical fault within the EGR system. P0406 is specifically a high circuit fault within the EGR valve and sensor (single unit). Again both trouble codes may be caused due to carbon build-up within the exhaust or intake passages leading to and away from the EGR valve, but more than likely the problem is not with EGR exhaust flow but rather an electrical fault, specially on a later model vehicle with lower mileage. Enough time may have not passed to collect the amount of carbon needed to throw a P0403 trouble code. You might get an insufficient flow trouble code P0401, but rarely a P0403 or P0406.
posted by SmogTips Support 04-27-2017 11:50 AM