1. In general, is it bad to idle for long periods bad (about an hour a day)? I did this on my previous SUV and it needed new cats a few times. I got the Cayenne less than a year ago and now I got these codes. I've read that it could be bad for the cats.

2. Replaced the gas cap to be sure it wasn't something simple, reset the codes with the Durametric. Where should I go from there? It seems like the problem could be various things? I am a pretty good DIY now on Porsche (4 between me and my friend).

 

Idling, for no beneficial reason, is pointless. It allows things (like cats) to overheat and fail. You have no air flow while stationary which is needed to cool anything that not water cooled by rather engine. I've been driving for over 40years, don't idle for any reason as it is simply a waste of fuel, and have never replaced a cat.

 

In my case, there is a beneficial reason to idle it...so lets just assume that.

I always wondered if it would do damage to the car, but thought a lot of people would have similar issues if they were to be stuck in traffic or the like (cabs and cop cars would seemingly have this issue?). So perhaps in the summer it is really bad, but may not be as bad in the winter? Perhaps I messed up my cats because of this.

 

The only way I have heard of cats failing is by running too rich too long. If you have Durametric, then you can see how rich your fuel mixture (AFR) is at idle. Anything lower than 12 is going towards too lean and anything higher than 14 is going towards too rich. Without any other facts, I would lean towards coincidence.

 

Limos, police cars constantly idle, they all seem to get hundreds of thousands of miles with no issues. So other than environmental, I don't think does much if any long term issues. I think back in the carburetor days, it was an issue, but engines runs so clean today, it seems it does not matter.

 

I found there were cracks in a few of the vacuum hoses (those ones that connect with a plastic Y-connector). Two connect near the back of the engine and one connects to the front intake area. After replacing those, the codes went away.

It may be possible that idling it for a long while played a role. Perhaps it got hotter than usual (since there is no airflow) and the hoses got hard, brittle and ended up cracking in the heat.