Showing P0455 on My 2006 XL Denali

Hi Guys

I'm showing a code of P0455 on my 6.0L Denali and was wondering what might be the first place to look for this (EVAP) Malfunction? Thanks Guys. Any help appreciated.

P0455: System Gross Leak Evaporative Emission. So many threads here about evap system problems and so many times members will try to piece meal and patch it and it gives them fits. That is why, when I addressed mine, before I got any codes, I just replaced the entire system. Many places to look for problems:

• Evap System Hose– The hose that carries the fuel vapor from the fuel tank to the intake manifold can begin to leak. This will cause the P0455 code. Look for either a tear or hole in it. This is one of the most common issues that causes the code
• Vapor Canister Purge Valve– One of the most common causes of the P0455 code is the vapor canister purge valve. It’s really easy to replace. There are a few clips that you need to find and remove.
• Faulty Charcoal Canister– The charcoal canister is often the cause of P0455.
• Fuel Tank Sending Unit– It is possible for the gasket that attaches the fuel sending unit to the gas tank to go bad. This can reduce tank pressure and throw the code.
• Fuel Cap– A bad fuel cap can cause the code to be thrown for sure. Often, you’ll get a message informing you that the fuel cap is off even when it is on. If you are getting that message, plus a P0455, it may save you a ton of time and money to replace the fuel cap and see if it goes away.
• Fuel Filler Neck– There is often a gasket where the fuel filler neck connects with the gas tank. This gasket can dry out, especially with vehicles that spend their time in dry climates. When it dries out, the fuel tank can’t hold pressure anymore.

About a month after I bought my 04 Tahoe Z71, I started getting the "Tighten Gas Cap" message on the DIC display frequently.

Then in November I dug into history of stored codes with my Zurich scan tool (found it under the "Chevrolet OEM Enhanced - ECM" menu):
P0449 Vent Valve
P0442 Small Evap leak
P0446 Vent Solenoid
P0455 Large Evap Leak
P0496 Large Flow during non-purge

Then, some of those codes kept coming back...
P0449, P0455 & P0496

Since the large flow during a non-purge cycle was the newest code to come in I tested the Purge valve with my Tech 2.
It was not responding to open/close commands.
Figuring it was stuck open, I replaced with a new Echlin purge valve from NAPA.

Cured the gas cap messages and there have been no Evap. related codes since.

So all the other codes pointing to the vent system were really caused by the faulty purge valve.

That was a cheap fix for me and I am really glad I didn't run out and replace the vent valve, filter canister or anything else!

That one purge valve part made it look like the vent system was the problem and there was nothing wrong with the rest of the EVAP system at all.

Tech 2 scan tool (in my case) or other bi-directional scan device can save a lot of time and money!!! Something to think about, if you don't already have one.

We just replaced the entire system on our 2005 Suburban 1500. It went from allowing me to put 7 gallons in the tank to letting me put 29 gallons in the tank... (it has a 31 gallon tank)

P0455 might not be a leak at all. If the purge valve on the intake is not allowing the ECM to pull a vacuum on the rest of the fuel system, the ECM will think it has a leak. I would recommend a purge and seal procedure to help determine what's going on.

I'll see if I can find that Wells tutorial and post it.

EDIT: see this link for some good resources:

This might help too:

GM "P0455 Large EVAP Leak" - What To Check When It's Not The Gas Cap

Yes, the Tech 2 helped me figure out my purge valve was not working quickly and easily. I could see no change in status (no response at all) when issuing open/close commands from the Tech 2.

It turned out to be the cause of all the other EVAP codes/issues (vent, large leak, small leak, gas cap, etc.)...
I was super happy this only cost me $36, took about 5 minutes to replace and I didn't even have to crawl around on the ground!!!