Hello. I have a 2002 Chevy Blazer and it goes uphill "very slow" and there is ALWAYS exhaust coming from the catalytic converter even when I am not in the car. When I put my foot on the gas pedal, I hear a "hissing" underneath the car. What should I do?

 

You have a 2002?
*Moved to '2nd Gen Tech'*
Your symptoms sound like a possible clogged catalytic converter. Rap on it with a mallet or your fist and see if it rattles. If so, replace it.
Second idea, remove the O2 sensor in front of the cat and see if it makes a difference in performance (it's going to be loud). If it does, replace the cat.

 

Check the coolant level.
Pull the engine oil dipstick with the engine running; does it appear to have foam on it?

Want to make sure the "hissing" is not coolant getting into the exhaust.