Vehicle is a 2004 Mercury Monterey, V6. Over the winter the injectors were replaced, and all 4 O2 sensors. It failed a state inspection, because the catalytic converter had been damaged, presumably from running lean prior to the injector problem being diagnosed. The new catalytic converter is in, but now is showing P0171 and P0430 error codes. What are the likely culprits? It seemed immediately after the install that some exhaust may have been leaking around one of the flanges between the exhaust pipe and the manifold, but all 4 bolts (2 for each connection to the manifold) seem very tight now.