Seeking advice in fixing P0420

All,

I am new to dfwmiata. XM_Rocks told me about this site on miata,net. Need advice on the following problem:

Got the P0420code over a week ago. Checked the miata.net forum and decided to get the intake cleaned. Went to Walmart and had it done (Gumout fuel sytem cleaning). Threw a blinking CEL during the cleaning process. I figured since the engine did hesitate some, it could have fooled the ECU to throw the code. Now blinking CEL is gone and solid CEL is back. My 10AE (original owner) has a shade under 40k miles. Coil pack replaced about 6k miles ago after replacing plug wires (NGK). Am still running on Bosch Platinum Plus 4 plugs. My question is should I try replacing the front O2 sensor or go ahead and replace the catalytic converter ?

This issue has been discussed several times on here. Personally, I'd do an 02 sensor and go from there. Good luck!