I have this 2006 Accord V6 with 90,000 miles that has the check engine light on with just one code stored, P0420 Catalyst System Efficiency Below Threshold (Bank 1). If I clear the code it comes back in 10 to 20 miles with the CEL on again. As suggested on another forum, I swapped the downstream O2 sensors front to back. (Not a fun job) So now I don't get the CEL on anymore (at least in the 60 miles I drove it so far) but the scan tool still stores the same P0420 code under pending codes. What does this mean? It will never pass a smog inspection while there is a pending code stored. Can I try one of those O2 sensor spacers on the bank 1 downstream O2 sensor?

 

The catalytic converter is most likely bad. The code setting in pending means it failed the first readiness test and if it fails again the cel will illuminate and the code will be stored in current codes. The engine computer is monitoring both sensors, the front sensor before the cc and the rear sensor after the cc. If the computer doesent see enough of a change between the two sensors it interprets it as a bad catalytic convertor. A leak in the pipe before the rear sensor can trick the ecu into throwing this code. Good luck