2003 EX Honda accord with 203000 miles. I am getting ready to replace the cat next week. I also noticed some "hesitation" from the car. I suspect carbon buildup. So I put hi-grade gas in it and the "hesitation" goes away but comes back. So I am thinking about using sea foam to clean the carbon build up from How To: Use Seafoam Motor Treatment - Focus Fanatics
Before the cat gets replaced. Do you agree with this or is there a better way for my Honda? That link mentions undoing a vacuum line; where is that on a 2003 ex Honda accord?

 

Suggest cleaning throttle plate and throttle plate bore. Also clean the IACV movement.

Blocked converter might cause hesitation.

What DTC's are set by the computer?

good luck

 

Gets P0420 code - cat efficiency below threshold bank 1
Don't want to replace cat and then try sea foam when that could clog the cat. Do you suggest the link as the best way to remove carbon buildup?
Where are the parts you listed: throttle plate/bore and IACV?

 

It may work and should not hurt anything to try. Check scottykilmer on youtube for videos on converter rehabilitation and repair. He recommends removal/soak/washing for worst cases. Cost of a new OEM converter justifies effort to fix.

Monitoring the secondary (aft of converter) O2 sensor performance is surest way to verify that converter is soure of P0420. I did this on 1999 Subaru Forester w/ persistent P0420 and found O2 sensor voltage fluctuating from 0.1- 0.9 v at frequency similar to primary o2 sensor. I replaced converter and fixed problem for about 2 yrs and experience a recurrence.

good luck