Q: Have a 2006 Lexus IS350 with a code of P0420 saying catalyst System Efficiency Below Threshold Bank 1

Hi Phoebe. Thanks for contacting us today. When the oxygen sensors have readings that are similar to each other, it is an indication that the catalytic converter is not working as intended. If the voltage of the downstream oxygen sensor decreases and begins to fluctuate like the upstream oxygen sensor, it means that the oxygen levels are too high and the Powertrain Control Module (PCM) will store the P0420 trouble code. This can cause poor fuel mileage, shaking and vibrating (as you’ve described) and poor engine efficiency. Since the O2 sensors or the catalytic converter itself could be the source of this problem, having the root issue correctly diagnosed by a professional mechanic is the best initial step.