I also had a leakiy cat. Seem slike my problem was that the seams were busting out and the air was leaking at the weld seams. Lucky for me emissions warranty covered the problem.

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1997 Silver AMG C36
OEM Advantegarde Grille
OEM Clear Corners
W210 Blinking mirrors
Eibach/Bilstein Combo
Zender Rear Lip Spoiler