My 01 Odyessy is reading code PO420 Catalyst system efficency below threshold.

Can someone tell me what this means?

In the chart I have it says that the associated monitor is:

Catalytic converter monitor

Could it be just oxygen sensor?

Sam

I cleared the code about 500 miles ago. it has yet to come back. so for now I'm not worrying about it.

I hope it is just the sensor(s)

My 01 Odyessy is reading code PO420 Catalyst system efficency below threshold.

Can someone tell me what this means?

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Lighthttp://kona.kontera.com/javascript/lib/imgs/grey_loader.gif
(http://www.obd-codes.com/p0420#) if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Some suggested steps for troubleshooting a P0420 code include:

Check for exhausthttp://kona.kontera.com/javascript/lib/imgs/grey_loader.gif
(http://www.obd-codes.com/p0420#) leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car (http://www.obd-codes.com/p0420#) but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.