hey everyone well i bout a high-flow vibrant Cat. for my 97 manual civic HX (D16Y5).

so when i installed my header and Cat. and when i went to take it for a test drive my car started to act weird. when im cruising my car it starts to slow down like if i'm stepping on the brakes and letting it go and then stepping on the brakes again. so I decided to remove the Cat. and test drive the car with open headers to see if it will still do it and it didn't it was driving fine, i was able to cruise the car with no problems.

Now when i call the guys that sold me the cat. they said that my car needs to relearn it's self because it has a high flow Cat. or i can get it tune. I never anything like that

Anyone know what could be wrong???