Dear Doctor: I have a question regarding the “engine light” that stays lit on my 2000 Mercury Cougar. The problem code was EGR valve-insufficient airflow. After replacing the EGR valve, the engine light again came back on with the same problem code. The EGR sensor was replaced. The engine light came on again with DPFE failure code. Do you think replacing the DPFE will finally fix the problem, or will this lead to a series of other sensor failures? On the other hand, do you think there is another remedy I am overlooking? Carl
Dear Carl: Very seldom is an EGR valve faulty. I see a lot of vehicles where the owners have replaced the EGR valves and other sensors that are not the problem. EGR flow problems relate to carbon buildup and/or faulty EGR-related sensors, such as the DPFE on Fords and VSV on Toyota’s. Carbon builds up in EGR passageways, especially the intake manifold passageways. Just because there is a fault code does not mean the sensor is faulty. This is where a technician can pinpoint the actual problem.