You could remove the cat, fill it with solvent over night and then rinse it out with a hose. This works very well but it's a bit of work. The easy way is to install one of these:

http://www.bigdaddiesgarage.com/mini-cat...

First best answer to Paul H. A lot of misinformation out there about P0420/P0430, but Paul cuts through most of it.

I have fixed about a dozen P0420/P0430 codes, most repeats and have never replaced a catalytic converter to do it. The most recent was also the easiest: my daughter's 2002 Sienna had a persistent P0420. I put a bottle of Techron in the tank and instructed her to get gas at Conoco most of the time. The code came back in 50 miles, went away and stayed away after another 50 miles. It has been about half a year now.

P0420/P0430 is officially "Catalyst system efficiency below threshold..." but is produced by the ECU when a downstream O2 sensor reports high and fluctuating levels of free oxygen where the oxygen should be low and steadier. If the definition were corrected to the more accurate "Excessive oxygen in exhaust..." nobody would ever replace the catalytic converter and the repair rate might actually improve.

Yet virtually all professional mechanics approach the problem by replacing the converter, and most of the time the code will go away unless this is a repeat. The new converter is just enough better to squelch the code for a couple of years, long enough for it not to be a callback, and the mechanic makes good money doing it. But eventually the underlying condition worsens and the new wears off the converter and the code returns. At that point a new OEM converter will usually work for a few months, and then the code is back to stay. It is a very common story.

The underlying cause of the high fluctuating levels of free oxygen in the exhaust can be caused by intake leaks, especially at the intake manifold gasket; exhaust leaks ahead of the downstream sensor, or anything that interferes with proper mixture control - dirty MAF, dirty injectors, worn out upstream O2 sensor.

Toyota has a TSB or Technical Service Bulletin on some 02-04 Camry models with the 4 cylinder engine to replace the converter with an updated model and reprogram the computer. The repairs were covered under the emissions warranty for 96 months/80,000 miles but yours is likely out of warranty.

You might be best to have the computer scanned to test cat converter function and for proper O2 operation, check for vacuum leaks, low fuel pressure or dirty injectors, exhaust leaks ,etc... ,,,,,could just need an O2 sensor or other issues with exhaust system leaks, etc.. Run some Seafoam or Techron in the fuel system per label directions to clean injectors.

If the car is running OK and does not have to pass emissions tests, many people just forgo any expensive repairs since it's not a critical issue and there are ways to get the trouble code to quit temporarily enough to pass emissions tests.