The most common failure is carbon build-up inside the EGR and cooler. This is inevitable over a long time period, but is significantly increased with city driving and short runs. Problems with the particulate filter can also cause excessive carbon build-up. Other problems are often related to mechanical wear. The signal fed back to the ECU about the valve position has to be accurate so any variations outside of the normal tolerance are detected by the ECU and error codes are then logged. The cooler bypass can also sometimes fail or develop external leaks.