I have recently fitted a stainless steel cat back system to my 2001 1.8 16v. Since doing this my engine light has now come on. I have put it on the ethos snap-on reader and it is coming back with a o2 sensor fault, erased code but light came back on after approx 1 mile. Any ideas ? The sensor is in the cat on the manifold so upstream of the new exhaust so I can't understand why it is faulting. Seems a bit strange that the sensor faulting coensides with the exhaust replace.