Chech Engine Light

[camlex]

Hello,

As previously posted, I got CEL and code came out P0420.

So i replaced Cat. Converter @ 279,XXX. Now i have 284,000 on my car.

Again CEL shows up. run code through Autozone : P0420.

While CEL on I was driving and light disappear.

No more Check Engine Light.

What happens?

please advise.

Thanks in advance..