Q: Is the catalytic converter clogged if it starts and runs but rattles kind of when it starts and doesn't accelerate very well?

asked by on October 27, 2016

If it starts and kind of rattles when it idles and doesn't accelerate very well as the catalytic converter clogged

My car has 152549 miles.
My car has an automatic transmission.

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This is a common description of a clogged catalytic converter. If it is the catalytic converter that is rattling, that means the honeycomb lattice inside the cat has come loose. However, this doesn’t mean the cat is clogged, but it could be. Checking for a clogged exhaust system is fairly simple. Simply put your hand in front of the exhaust outlet and see if there is plenty of air exiting it. If you are not sure it is enough, do this with a good running car that does not have a modified exhaust. It should be as strong as a typical fan in your home. Following this, have someone rev the motor up and see if the air flow out the exhaust increases significantly. If the motor does not rev easily, floor the pedal and see if the RPM’s will rise or stop and not go any further. Revving a motor in this fashion should quickly rev to red line with little effort. If it sluggishly revs, this is another sign you have a clogged exhaust.

If the motor won’t rev, the next thing to do is create an exhaust leak as close to the motor as you can, then rev the motor. It will be very loud, but if it is now able to rev, you have confirmed the exhaust is clogged.

After doing these simple tests, we can get a good idea if the exhaust is plugged. While the catalytic converter is the most likely culprit, other things can occur to clog an exhaust system. The rattle could be another part of the exhaust such as a metal heat shield or broken exhaust hanger.

A lack of acceleration can be caused by a large variety of reasons other than a plugged exhaust. Follow the steps I laid out about and you should be able to come to a conclusion.

To avoid any unnecessary repairs, I recommend having the loss of power inspected by one of our mobile technicians at your own location. Be sure to notify the technician about suspecting the exhaust is plugged.

Good luck!

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