EGR Woes Continue

My '96 4cyl Contour has been kicking out a P0401 and P0420 codes (Low EGR Flow and After-Cat O2 reading out of tolerance), so I finally bit the bullett and installed a NEW EGR (it was a PITA to remove, so I didn't want to try cleaning it first) and a new After-Cat O2 Sensor.

SAME Problem, same codes. The EGR is controlled by a Solonoid Valve and an Exhaust Pressure Differential Sensor (both mounted on the firewall) accoring to the Haines Manual.

Has anyone had either/both of these go bad and kick out those codes?

I hate to spend another couple hundred bucks on parts without knowing if they cause that smptom.

The P0420 code(...Efficiency below threshold) is set by the lack of large enough difference between the upstream and downstream O2 sensor readings. The downstream O2 sensor is ususally NOT bad and is often erroneously replaced because no one looks at the sensor output(scan or voltmeter test). The best bet is to make sure all the connections are clean and then graph the up and down stream O2 sensors. The downstream O2 sensor serves no purpose in engine management - it simply checks the converter efficiency. Your cat is probably NFG. Go aftermarket - much cheaper than OEM.

DON'T GUESS - TROUBLESHOOT

IIRC, the 401 can be caused by the DPFE sensor. I believe thats the code I got when mine went. I'm not sure if the Zetec uses the same one as the Duratec(I'm almost sure they do), but they are notorious for failing in Contours. They are metal and corrode, eventually failing. Try to find a plastic one if you replace it. They usually run around $50.