Welcome to the forum.
What is causing you to say you have to find the root cause of the CEL being illuminated? Is the car stalling or lacking power?

I've a '97 accord with 280,000 on it and its been running well enough with the check engine light illuminated for 50,000 miles.
But to solve the CEL problem, cut a half inch square of black electricians tape and cover up the CEL.
I did that once, and it started no end of interesting conversations with quite a few passengers. Give it a try.

 

That won't work if he has to get emissions testing for his license plates...

However, what is the actual error code from the scanner? Knowing that code, it's still not automatically "buy this part" or something simple. A code that indicates a problem with the secondary O2 sensor might be the sensor itself, or it might be a bad wiring harness that's not making good connection to the sensor.

Your 4-cyl engine only has 1 bank. There's a primary and secondary O2 sensor, it's typically the secondary one that provides the measurement to say your catalyst isn't working. But like I said, it might be a flaky wiring connection since you've already replaced the expensive parts and that didn't fix it.

 

Like Jim said, your car should only have two O2 sensors. One in the manifold, and one in the Cat. The 1 in the manifold does the primary mix reading needed for the ECU, while the 2nd O2 sensor just reads the cat. Since you just replaced your cat, it shouldn't be damaged so soon. I'm not sure about your 2003, but on the 98 thru 02's, there were 2 different cat O2's available, and you needed the right one for them or the check engine light (CEL) would come on. In my case when I replaced the cat on my 99 Accord, I reused the original cat O2 sensor, and had no issues (no CEL) light. My cat was bad, but the sensor was still good.
If you still have the old sensor, I'd re-install it, clear the codes and see if that code comes back. I'd also check the wiring to it (look for any breaks), and make sure it's actually plugged in.
It's also possible you're reading the wrong code. I only say that since you replaced the gas cap.