P0420 can have several reasons; leaks in exhaust ahead of converter, marginal primary (ahead of converter) O2 sensor, marginal secondary O2 sensor behind converter, or worn-out converter. If you have access to a scanner than can monitor O2 sensor performance, you may be able to diagnose the problem.

Honda converters seem to be very robust so check carefully before discarding your old unit.

good luck

 

About extra ATF drain/fills...
The torque converter doesn't drain, so you only drain/fill about 1/3 of the total fluid. Since it smelled bad & there was gunk, I'd do a couple more drain/fills to remove more of the bad fluid.

Good choice to use Honda ATF.

Oxygen sensors actually measure oxygen DIFFERENCE from inside vs/ outside of sensor. So exhaust leaks blowing onto the outside of the sensor can throw off the measurement. Same thing with oil or gunk dripping on the sensors. Check both sensors, because the front sensor has some control authority over fuel mixture, and if that's off, the converter might have more difficulty operating within EPA limits.

 

Thanks for the advise on the 02 sensors. Ill be going to the shop today to figure it all out and probably do another fluid change.

TEXAS- what do you mean when you say marginal?

 

Marginal O2 sensor....
Common way for an O2 sensor to fail is it gets more & more sluggish in measuring. So it doesn't correctly measure things that are changing fast.

It might be just sluggish enough to throw off the measurements that trigger P0420 but not bad enough so the ECU recognizes it as a bad sensor.

 

If O2 sensor were totally non-functioning, the ECU would detect and set a code. So sensor must be degraded, but still functioning enough not to set O2 sensor fault code, but badly enough to cause problems. I know sensor degradation can happen w/o a code can happen based on fixes posted by others, but in context of P0420 this is speculation. Manuals don't list O2 sensor as a potential fault.

good luck