Different Smell from Hot Engine - Cats?
 

I recently had CEL for drivers side air sensors, they were not persistent, but periodic, cleared them with Duratec. I have replaced all the sensors about 5 years ago. I cleaned the MAF (which was dirty) and have not seen codes for a while. But I have noticed a different smell after driving the Boxster S plus a new noise at idle

I do have an unsteady idle before the engine warms up - car seems to have normal power it is not running hot.

One time I had cats go bad it was on my mustang - no power, engine ran HOT

I had the cats go bad on my land rover but that they were just falling apart on the inside made noise at idle

Any thoughts here - Car has 103,000 miles and is tracked once in a while - do the cats rattle on Boxsters?

Not sure about the uneven idle but years ago, I noticed a different smell after driving and found two of my axle boots cracked and flinging grease everywhere, including the exhaust.

No cats do not rattle, if they do you got chucks of the material inside degrading. Maybe your smelling leaking fluid dripping on to the exhaust or hot surface? Check fluid levels regularly.

+1 to grease from an axle boot burning on the exhaust. Look for dark residue on the exhaust where the axle crosses. A possible leak that would not have the classic 'burning oil smell' would be fluid coming from the power steering reservoir or overflow tube.
While the cats themselves do not rattle I have seen the heat shields come loose and rattle.

It's not a hot coolant odor is it?
I got that shortly before my water pump went several years back..

It's more frequent for a cat to plug and do what it did on your mustang than it is to see one fall apart from the inside like your rover.

Your smell may be a small oil leak (spark plug tubes are common culprit, drip into exhaust piping).

What does the noise sound like? A rear end rattle could be a lot of things.

I'd get the car up on jacks and have a look around to figure those two things out.

A fluctuating idle can mean a lot of things. If it's only with cold startup at idle, it's likely a small vac leak. Do a manifold vac test and see what that shows you.

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