I noticed after seafoam test that there is a little tiny leak on the exhaust manifold gasket. but its small. Im running a big downpipe 3 inch on my s40. is the 420 code for the front or back? ive looked it up and people have been saying both.. 2000 s40

 

the P0420 code is measuring catalytic converter efficiency by looking for a delta between the two O2 sensors. When you look at how O2 sensors can fail, it can be due to a bad heater element or the sensing circuit. With that said, when you have an exhaust leak or a free flow exhaust, that can mess with the dynamics of the sensors. You may want to read up on what a Cel-Boss does - (ie removes the sensor from the direct airflow stream ). If you car is running reasonably well and you are sure you're cat is in good shape, that may be a quick trick to try.