ECU giving CAT at Capacity code?
 

Hey my ECU is giving Catalytic Converter reaching Capacity code and making my Check engine soon light stay on. I have normal gas mileage and seems pretty normal. I have a Air Intake on which got rid of the airbox and has the Air temp sensor hanging next to the intake, ( so I figure the air going in can't be much different in temp). Also I have a straight thru muffler but kept all CAT's amd resonators, so maybe a little more flow. So, I am not sure how to fix. The Autozone guy said it happens from filling car with gas while running, ( I never do that), and can be from not letting the car warm up before driving, ( somewhat guilty). But the kicker is that it came on after driving 2 solid HRS. at 90 MPH down to Silver Dollar Raceway for Nopi event this past weekend, so doesn't seem to add up. Any Suggestions, new O2 sensor, or new pre-Cat or Higher capacity Cat? What do I need to do?

I need to know what code they said it was. A cat code would not show for a refueling problem that would be a evap or fuel tank level sensor code.

Oops code is different
 

Ok, I went back and got the code P0421, it reads the Warm up catalyst is below threshold in bank 1 . I am not sure what that is, but I want to fix it. I am guessing O2 sensor, but if you know what that is then cool. I will wait before doing anything. Thanks, Chris

P0421 could be caused by a previous misfire that has damaged the converter , damage to the wiring ( sometimes the heatshield on the converter will chafe the wires)for the rear O2 sensor or the sensor itself . These are the most common cause I have found.

I fixed it
 

Hey I fixed it, I asked around and all these garages and Pepboys were like well that code could be many things and without our $80.00 diagnostic we won't be able to tell for sure. I was like the O2 sensor is $79.99 and they need to be replaced around 80,000 miles and I have 84,000 anyway, so I took a chance. I got a new front O 2 sensor, we have 2 and I got the one before the CATs. I suck though, they were like do you need the O2 sensor tool that Autozone lets you borrow free with a $15.00 deposit and I was like no thanks it is right on the front and should be easy. Man, I had to take off the Manifold heat shield, and couldn't get that damn thing to budge cause everything I had was slipping off. Luckily some guy just happened to walk over and had a big wrench that fit it and it was long enough for some goog leverage. I then put everything back together and took the Negative battery post off for about 10 seconds put it back on , got in and turned the key and pumped the brake 5 times and when I started the Check engine light was out and when I drove had a tad bit more getty-up!!!!!!!!