I was getting a P0420 (low catalyst system efficiency) on my 2002 Honda Accord with 186K miles.

In the past I had success clearing this code using products that you add to the gas tank,
like Dura Lube, but this time I used a can and it did not clear the code.

I saw a couple of YT videos showing the use of citric acid to clean a CC, so I tried that.
And it worked.

I removed the CC from the car, and put a rubber stopper in one end.

I put 1/4 cup of citric acid powder in a gallon of water, and heated it to 160F on the stovetop.
I poured it into the converter, put a second stopper in the top end, and swished it around.
Let it sit for 30 minutes, poured the solution out into a bucket.

I repeated this 3 more times. It got out a lot of dirt.

I put the CC back on the car. The first 50 miles or so, the P0420 returned, so I thought the
cleaning process was a failure. But after 50 miles, the code has remained off.

The car passed NYS inspection. All monitors completed.

Starting next year the car will be exempt from emissions testing, as it will be 25 years old.
But I didn't want to buy a new CARB compliant CC, minimum cost for a Walker is $500 online.

It remains to be seen how long the code stays off.