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Error Code P0455 after Thermostat Change

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I just completed the thermostat replacement on my 957 08 Cayenne S and everything went well. Except now, i get a the code 0455 - Evaporative emission system leak detected (Large leak) No signal/communication. Did I miss connecting a line or is it related to the gas cap? Anyone know?

Edited by bobo83
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Yes, likely something loose, not connected, or damaged.


Fault 0455

Possible cause of fault
- Tank cap not tightened correctly, faulty or missing
- Mechanical fault in EVAP canister purge valve (does not close fully)
- Leakage diagnosis pump leaking internally (membrane) or externally
- Mechanical fault in EVAP canister shutoff valve (does not close fully when power is supplied)
- EVAP canister shutoff valve connections leaking
- Tank system (including pipes) leaking
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Thanks for the quick reply. Do you have a diagram on the 957 engines of the fuel system for me to trace? I just pulled all the engine covers again and double checked all the connections as well as the fuel cap. So I guess I'll see what happens tomorrow.

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Must be something up front that you disconnected when you did the Thermostat. Rarely does the gas cap set this one off as it is the large leak fault. Usually for the gas cap it is the fault for the small leak detected unless you completely forgot to put the cap on.

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Will PWIIS tell me exactly where the leak is? Is there a fuel line that runs under the intake manifold?


No diagnostic tool will tell you "exactly where the leak is", only that there is a leak, and the possible causes (see Loren's post above).  It is up to the diagnostic technician to locate it by a process of elimination.

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Hmm, ok. I'll check the lines 1 more time. I have a feeling I know which line it is, however, not 100% sure. Loren or JFP, do you have a smoke test DIY?


You can probably find something online, but be aware that a smoke test machine is not an inconsequential purchase, but you might be able to rent one.  This might also be a good job for a well equipped shop to handle.

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Did you guys figure out what happened? I did the same exact service, swapped the thermostat and now am getting this code. Only thing I suspect is the vacuum fittings on the back of the manifold. Can't imagine anything in the front.

All help is greatly appreciated.



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  • 2 weeks later...

I'm still having issues with this. I reconnected all the hoses and this is what I've been thinking so far.


1) I found oil in my intake manifold, so I'm assuming that possibly my AoS is going and this could cause P0455?

2) the line vacuum one at the back of the engine is cracked and I can't see it, so I might just replace it.

3) the oil residue that I find around the intake manifold could be caused by old or worn gaskets which need to be replaced.


Any ideas would be great!

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  • 2 weeks later...

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