How to Diagnose EVAP System When Check Engine Code Says P0455 or P0442

If you get a check engine code referring to the EVAP system, but don’t know how to processed. This article will give you tips on how to diagnose and repair those problems.



Check gas cap


The first thing we need to do is check the gas cap. Make sure it’s not too loose, then pull it off and check the seal. Make sure the seal isn’t cracking. When you have a gas cap leak where it’s the seal cracking, you’re going to get a small system leak.

Also check the filler cap, see if there’s any rust buildup. If there’s anything rusty there, you can sand it down with a little bit of sandpaper or emery cloth, or replace the gas cap.


Check the EVAP purge valve


Under the hood we have a purge solenoid, what that does is brings vacuum to the EVAP system. You get the vacuum through the intake manifold as the engine’s running and there’s an electrical connector attached to the purge valve. If these leak a lot of times, you’ll get a different type of code, but sometimes you can still get a small leak or large leak code. In that case, you just got to replace this solenoid. It’s very easy, just disconnect the hoses, disconnect the connector and put the new one in.


Check the EVAP vent solenoid valve


The EVAP canister vent solenoid is leaking, that’s going to cause a large leak or a small leak. In our case, it’s a crack in the hose, and then the solenoid which is a electronically controlled valve also has a crack.We have to replace it.


A common problem is these valves getting clogged up or leaking not just visually, you won’t be able to see this all the time. Sometimes it’s internal , debris and sand and other contaminants will get into the valve and just cause it not to close properly, and that’s going to give you a check engine light. You can test this system is with a smoke tester, if you don’t have a smoke tester, you can do the best, check for anything obvious, it’ll probably help you out.


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