Reset the codes and now PO421 has come back. What is causing this?

We took the car in for emissions and the cat and sensors were bad. We replaced them with Exact OEM direct fit parts and did two recalls on the car( PCM reflash and fan controller) and replaced the spark-plugs, PCV valve, pulley belts, valve cover gasket, air filter, oil-change, and fuel-injection flush service and a fuel injection hose we found was broken. I replaced the front and rear O2 sensors and the catalytic converter. We reset the codes, and drove the car approximately 81 miles and the service engine light came on again. Took it to autozone and came out with the PO421 code again. They said we either didnt get the converter/rear sensor on tight enough or its an exhaust leak. The service engine light came on aprx. 10 min after going through the car wash. If the converter/sensor wasnt tight enough is that going to cause the problem? Could water get into the converter? Or is it a exhaustleak somewhere? What do you recommend now? Also when I turned into autozone, the car stalled, but started right back up. Thanks