Just installed new exhaust manifold back exhaust on a '98 2.5RS. The engine is not original, but is a JDM DOHC(hydraulic) 2.5(I've heard many times that there was no JDM 2.5, but the one I got came from Japan). The engine didn't have an EGR port in the head (I had to use the original intake manifold), so the EGR port on the manifold is plugged. The engine has been in the car for3K miles now, and has had an P0420 code since 100 miles. I had the new cat to install, so I'd figure I'd wait to see if the 0420 would go away with the new exhaust setup.
Yesterday I installed the exhaust; Random Tech cat, 2.25" mid pipe, and Prodrive muffler, which has 2.5" piping. The cat was actually designed for a '99, and I had to extend the O2 sensor wire using 18 gauge copper wire and butt connectors; the original sensor has 22 gauge SS wire. I reset the ECU, and at approximately 100 miles I CEL'ed. I figured that it was the good old P0420, but it scanned P0400, EGR malfunction.
What could have caused this code? The EGR code never occurred before the new exhaust, and I was wondering if my O2 sensor wire extension could cause this code. Would the new lack of previous exhaust back pressure have anything to do with it? Thanks