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OBDII Error: P0455 - 455 A large leak has been detected in the evaporative system...?


NAXJA Forum User
Garden Grove, CA
Hey gang,

Borrowed a code reader from the local AutoZone and found only one code: P0455 - EVAP Emission Control System Leak (Large).

Any ideas on what I should look for under the hood? What do you think needs to be replaced? Any ideas would be helpful. I'm very new to this vehicle/engine. Thanks!
Re: OBDII Error: P0455 - 455 A large leak has been detected in the evaporative system...?

Check your gas cap.
hey i work at an independent repair shop and see quite a few jeeps esspecially newer xj's come in with evap leaks. If you know someone who has access to a snap on scanner you can force it to run the evap test if you have the jeep software. this is very helpful to know if its fixed.

first off have you filled up the car with gas with the engine running? that can cause a major leak fault. if the answer to that is no which it probably is then in my expirence with xj's there are 4 things that i see regularly on xj's setting evap leak faults. the first is the purge valve it up near the right shock tower on a bracket. the next is the ldp pump its a big round thing next to the purge valve. the vent tends to stick open on these. I have had 3 97 and newer xj's have a major leak fault to find out the ldp pump is leaking then run the test and have a minor leak fault caused by the purge valve. On a few others i have seen the evap hoses by the test port crack. Also check the evap hose at the canister underneath the truck.

those are the normals.
Re: OBDII Error: P0455 - 455 A large leak has been detected in the evaporative system...?

Is there a way to actually check if it's holding vacuum? Or will this turn into a game of "replace all suspect parts and hope for the best"?!
Re: OBDII Error: P0455 - 455 A large leak has been detected in the evaporative system...?

I was gonna say gas cap, but it already was.

They cause problems sometimes.
xj6.0 said:
Also check the evap hose at the canister underneath the truck.

I don't remember the exact code number, but I had the same problem. It was a connection at the canister. Get under the Jeep and physically inspect the connections (Driver side, just behind passenger door, inside the frame rail). It will have obvious cracks. If there aren't any, follow the black tubing toward the engine and check the forward connections. If anything else, you can rule out one possibility.
from what i've seen ussually cracked lines will set a p0456 minor leak fault. however if there are multiple minor leaks due to multiple cracked lines or other items then it could set a major leak fault.

I mentioned this a bit in my first post about seeing 3 different xj's in the last 6 months with both a leaky purge valve, and leak detection pump. all these things are very common on late 90's early 0x's xj's along with cracked lines. Basicly when it comes to jeep evap the whole system is junk and is very proan to having leaks and failures. This is the case with most chrysler evap stuff.

at least the xj is one of the easier systems to diagnosis. as for you question about holding vacuum. its not that simple certian critera needs to be met. without a scan tool you will manually have to jumper items to make the test run. Mainly because the purge valve is opened at a certian time but its easy to bench test. then the pump runs at a certain time and the vent opens at a certain time ans so on. I tend to use a combo of the scan tool forcing the test to run and a smoke machine and i ussually have the truck diagnosised in about 20 mins. The nice thing about an xj is that everything is very easy to get to and remove. Not like some cars. try doing a cansiter on a late 90's early 0x camary sometime you will want to hang yourself with an extension cord.
one step foreward.... two steps back! Leak detection pump causes leak that wouldn't be there without it! All so we don't wonder IF the system is working properly-- instead, we know it's not! Just think, that pump probably raised the cost of the vehicle $1000.00 to make sure that only the rest of the world lets their gas vapors evaporate!:doh:
Re: OBDII Error: P0455 - 455 A large leak has been detected in the evaporative system...?

Ok, thanks guys...I have ONE more question!

My fuel mileage is about 13mpg MAX, on a 2001 XJ with 64k miles. Is this because the CEL is causing it to run in "closed loop/limp" mode, or could this be a direct result of an evaporation system issue causing the poor fuel economy?

The poor gas mileage was the reason I started looking at this problem to begin with. Thanks! :)
"open loop" not closed. If it's in open loop, your MPG will definitely suffer. I don't know if OBD ll will put you into open loop for an EVAP code though. Personally, I would think it's smart enough to know an EVAP code would not affect driveability, and be programmed to allow closed loop.
I'm not too sure its the gas cap here....usually when its the gas cap, it will throw a code for a small evap leak.....atleast in my experience, thats what ive seen on other cars.

A major leak fault usually indicates you have a bigger problem (badly cracked lines, or a faulty part).
an evap fault in a obd2 car will not cause a limp home mode or open loop situation. in most cases the fault will have to occur twice before it turns the light on for some evap faults.

your poor fuel ecomony may be caused by something else.

I've been working on cars for a quite a while now. In my expirence a major leak fault will be caused by a loose gas cap or multiple minor leaks. Minnor leak faults are caused by cracked lines and leaky purge valves stuff like that. Evap leaks are trickey to diagnosis on your own because you normally need a scan tool and a smoke machine to do it properly.