I was talking to a friend about my hissing sound at the back of the engine....and he says that I should check out the Cat.

Now -- I have had a long oil leak via bad stem seals before I bought the car and I've personally seen the cat glow purple one night.

With the hissing, poor power, bad mpg and the prior oil leakage....what could be the chances my cat is done for?

Is there any way to test this or is it just a replacement job?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k