Added a can of Seafoaf to half a tank and filled with 91 oct, no ethanol. Entered Interstate immediately and after 10 minutes at 75 mph the CEL came on steady. No difference in performance so kept on. 2150 miles stopped and had code read. P420 & P430 both set. As said car performing well with and without AC on. No driveability issues. Several 100 miles to destination before reset both codes. Tokk a 16 mile drive and no CEL or other issues other than low idle/stumble.

Anyone experienced any like problem after using Seafoam or other fuel additive?

 

Those are both codes for cat not working properly. If the seafoam loosened up a bunch of junk in cylinder, where would it go? Out the exhaust and into...the cat.

My guess is cat is now coated with some loosened carbon, making it function inefficiently. Hopefully it will clean itself from the intense heat it generates. If not, seems like a prime candidate for the acetone cleaning procedure featured in many youtube videos...

 

Do you have a OEM cat? I've had no issues using Amsoil fuel additive and stabilizer with a EVS Tuning high flow cat.

 

OEM Cat in play. 50 miles today after reset cel p420 & p430 codes 2 days ago. No CEL and car is running great. Suspect Seafoam did it's job and 250 miles of 4k + RPM added enough heat to clear the carbon contamination in the Cat.