So I figured I'd post this as a bit of reference info for future troubleshooters.

Around thanksgiving I spent the better part of a day giving my 991.1 (2014 C2S) a thorough bath, inside out.. One of those you do maybe a couple times of year, probably irritated my nosey southern california neighbors in the midst of the draught what I wasted so much water in the process. (doesn't matter now, we are dumping water out of the reservoir after the last month of record rain and snow).

In any case, as I fired up the engine to move back into the garage, the instrument panel lit up with yellow.. The message seemed relatively benign and not a dreaded Red.... so Initially without researching, my first thoughts were that it was related to getting something wet such as ignition wires etc. Wouldn't be the first time I got one of these messages and it quickly went away.

So drove to work and realized it wasn't going away.. Did a little research and found it was the P0456 which cryptically seems to be some kind of minor vacuum leak in the emissions system, probably related to vapor recovery. I googled around and a few people indicated that it was fixed when the dealer replaced the gas cap. I made a mental note that maybe I'll order one of the chrome ones from the dealer.

I wasn't really sure if the problem was causing any issues. Performance seemed ok, gas mileage seemed ok... Although I did seem to feel that it was slightly rough at idle for a couple minutes when the engine was stone cold. I had never noticed, but can't say for sure that this was related.

Was right around the holidays so I didn't really have the time to get the car into the dealer, and since it didn't seem all that critical I do what I always do.. I procrastinated and put it off. Over the christmas break I decided to dig into it, and got my durametric out, and fiddled around and reset the condition but he fault has hard and immediately came back.. Drat.. means I'm going to need to take it in to the dealer.

A couple more weeks of procrastinating and when I was refueling, I spilled some fuel right around the gas cap. And in the process noted that there must be some kind of drain hole in the gas cap cavity since I could see fuel on the ground by the tire. (the fuel I spilled was fully within the gas cap cavity/lid and didn't run down the side of the car. Hunh.. I thought. .never noticed that there was a drain there.

Got back in the car, drove off... When I got to my destination I looked at the instrument cluster and I had a low tire warning...just a tad low so I cleared it.. And low and behold.... no other faults......!!!! first time in two months it went away.

So... my conclusion was that somehow clearing the fault was related to spilling a little fuel in the gas cap cavity and the fuel running through a drain tube. Maybe when I washed the car I got some grit or something in the tube that was cleared by the fuel acting as a solvent..

I'm also not sure if there are other things that feed that drain.. When I did the major wash job on the car that triggered this mess, did have a lot of leaves and these funky tree nuts that were all gunked down in the corner of the window/hood.. where the wipers are. I know there is a drain there somehow so maybe that caused the blockage. If I had it to do again, I'd probably use a little compressed air and blow things out.

Also note that the slightly rough idle.. maybe rough idle isn't the right term as it was barely noticeable hasn't happened either. That may or maynot be related.

In any case, I haven't had a recurrence in the last couple of weeks. So it would seem that a drain blockage can trigger the P0456.

 

Have you refueled your car with the engine is running, that will also throw the P0456?

 

No of course not...

I would presume even if someone did that it would clear itself once the cap was reinstalled.. I've gone through weekly refeueling and manual resets of the faults via my durametric..


I did find out once that low octane fuel will set off a red ECU "drive on" fault.. I refueled at a station that must have misfilled their tanks with lower octane or was contaminated.. refilling with good fuel cleared immediately the fault..

 

Very interesting post. Emissions regulations on modern cars are all monitored to the ECU and a plugged up gas vent hose is one of them. Sounds like you solved the problem. Thanks for posting.

 

No that is not my experience. Once the fault is present, it must be reset. It does not go away by itself. Personally, I don't fuel my cars with engines running, but I have seen others do so.

 

I can tell you without a doubt P0456 related to a vent plug goes away on it's own. I can't tell you if it occurred immediately after startup, or at sometime during driving (because I wasn't paying attention).

 

This Is interesting, I have the same code and most threads lead to to faulty gas cap

 

Thanks to the OP for posting this. Recently driving across 'America's loneliest Highway' my CEL came on. Due to my remote location, checking codes wasn't an option...naturally freaked me out a bit. Once i got a chance to pull codes, P0456 was the culprit.

Posting this for future reference. Once i read the original post on this thread I gave it a try. I poured some hot water inside the area surrounding the gas cap and sure enough there was blockage - the water would not drain. I gently used a clothes hanger (all i had with me) into the drain (maybe only 1/2in) and the water immediately drained. Started the car, CEL gone! Don't ya just love it when it's "this simple". Driving across the desert with that friggen CEL light on at 11:00pm was no fun at all,. Thankful for this post as "simple" as it was.

 

Glad it was helpful..

 

thanks, will try clearing the drain plug tomorrow. I have also filled my car with the engine on... hope it's reversible if that is the cause

 

Hello there, everyone.
That hasn't been my experience. It is necessary to reset the defect after it has been discovered. It will not go away on its own. I don't fuel my automobiles while they are running, although I have seen others do it.
turkish citizenship

 

in my case I had to replace the purge valve and all was fine again. Thanks to my local indie mechanic.