So my cat on my TL vrx is smelling like rotten eggs and smells like burnt plastic after a spirited drive.

I'm thinking if just replacing it with a hi flow cat, what size diameter is the piping? Would it be cheaper if I bought one off ebay and took one to an exhaust shop or just get it all done at the shop?

Standard cats flow plenty.....just replace it

explore possible underlying causes and eliminate them

edit add : my understanding is that if you have been running lean and then run rich the rotten egg smell is indicative of a functioning cat. However if the engine continues to run with a faulty mix, in time the condition can lead to catastrophic cat failure. (melt). The causes of faulty mix are varied. O2 sensor, misfire, ecu problem, and whatever causes that failure and so on . Unless the root cause is identified any replacement cat will only, in time, fail.

The 'rotten-egg' smell is usually indicative of petrol with a high sulphur content and may have nothing to do with the condition of the catalytic converter.

Hmmm alright well i've had a look under the car, there is evidence of an exhaust leak on both sides of the flanges, could that be the cause of the smell? The cat could be fine if thats the case

I think this helps t explain it :

http://www.answers.com/Q/How_can_you_tell_a_catalytic_converter_is_bad

"The rotten egg smell means that the catalytic converter is working properly.

"It's (the smell) hydrogen sulfide or H2S. It does not exist in the fuel or oil, which is why you can't smell it at the gas station. It does, however, come from the gasoline. Almost all gasoline contains organic sulfur compounds with sulfur levels ranging from 30 to 100 parts per million, approximately. "As the fuel burns, the organic sulfur compounds break down into simpler compounds. If the engine is running lean (more air than is needed to burn the fuel), the sulfur is likely to be SO2, or sulfur dioxide. The catalytic converter can change this into SO3 (sulfur trioxide), which then reacts with the catalyst to form sulfates on the catalyst. Over several minutes there is a build up of sulfates. If the engine suddenly starts to run rich (not enough air to burn all the fuel), such as when you stop at a red light, climb a steep hill, or brake hard to slow down, the chemistry changes. Now the sulfates are unstable and they react to form hydrogen sulfide, which is expelled from the exhaust pipe all at once. The high concentration is the reason for the bad smell! This effect is a common problem with new vehicles because the catalyst is working at a very high efficiency. However, as the catalyst gets older, the activity drops. Fortunately for us, one of the first reactions to be affected is the formation of hydrogen sulfide. So as the catalyst gets older, the smell will go away. It should also be noted that many catalysts never smell bad. One reason might be because the vehicle doesn't run lean for much of the time, or perhaps the catalyst has an extra component that reduces the production of hydrogen sulfide. "Next time Eggbert and Shelley stop behind a vehicle that smells bad, they can be grateful that it's at least not contributing much to atmospheric pollution." Thanks to this month's Whizard, Dr. Gordon Bartley, a senior research scientist in the Emissions Research Department of the Automotive Products and Emissions Research Division. Bartley specializes in the development, aging, and evaluation of vehicle emission catalysts. "

iow look upstream of the cat