I have a 2004 L-300 with a P0420 low cat efficiency code. My computer shows a normal upstream voltage rise/fall at idle. The downstream shows a steady state but pretty low. When I press the pedal and raise the rpm the downstream starts tracking the upstream sensor. I replaced the sensor as the place I bought from (junkyard auto sales) couldn't confirm the correct sensor installed when repairing the problems it found when they got it. There is the tiniest exhaust leak blowing out a pin hole at the forward weld on the cat.
Is this a clear cut suck it up and replace the cat? Anyone ever have this code to fix?