This one is stumping me…. the car is throwing a P0420 code, but it’s running fine. No smell, no smoke, no power loss, misfires, etc. It’s running very normally. I recently had all the spark plugs replaced as regular maintenance and then a few days later this code showed up. I replaced the upstream O2 sensor and the code came back. Would this be a cat problem? I’m used to those smelling like rotten eggs. Spark plug harness would cause misfire…. Should I replace the downstream O2 sensor? Anything else I should check before I end up disassembling half the car? Our local Jag guy isn’t reputable any longer, sadly. I’ll have to take it to a general shop.

 

Bank 1, change both sensors and don't buy cheap internet ones that nearly always fail or don't work initially. Use Denso or Bosch