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My Day Yesterday And Today... Could Use Some Help.


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Ok last few days.... i was on my way home and my truck LCD said to tighten the gas cap... i was like uhhh ok i guess i need a new gas cap. so i buy a cap at o Reilly’s and delete the code. and on my way home i stop to get some fuel, shut the truck off go pay get fuel start the truck and the check engine light comes back on... damnit maybe a vacuum leak somewhere but then it tells me an engine oil is low so i get home (i live 1 mile from the gas station) check the oil yes it was low. So today left home earlier stopped had the oil changed, and had jiffy lube try to tell me i needed my front diff and transfer case oil changed...by the way i have a 2wd. someone tell me how to change the oil in the transfer case and front diff on a 2wd. so i leave there and the check engine light is on still and then my truck tells me to tighten gas cap again. Making me mad. I hate having lights on my dash. Does anyone know what it could be? Anything would be great. My buddy is bringing me his OBD2 reader at lunch and ill have the code then. This is driving me nuts.



Any help would be great.

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has to do with the evap valve. could be the plastic line on it that goes over your intake manifold. if you removed the covers you would see a solenoid on top of the intake mani next to the throttle body with a small green cap . you may command it to open with a tech2 to check if its working . most likly a cracked plastic line though they are somewhat common to crack and suck air so it thinks the tank isnt sealed.

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yes.. i hate j-lube but i did not have much of a choice... i should have told them to go ahead and do the front dif and t-case. that would have been funny.



chase thanks for that info i will look at that for sure. only thing is i dont have a tech2.... could it be a random vac line?

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i should have the code here soon. but i found some good info for anyone else looking.



The dreaded GAS CAP code!!!


I see many posts here about how to fix ( the dreaded GAS CAP) EVAP codes. Today I am going to educate you as to the functions and diagnostics involved in Evaporative Emissions issues and what the codes mean.

Lets start off with the whys.

The purpose of the Evaporative Emission system is to limit the amount of Hyrdocarbons released into the atsmosphere.

Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid OFF, fresh air is drawn through the vent solenoid and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

So in a nut shell if the control module senses a leak in the system that will allow more than a prescribed amount of fuel vapors escape, the module will illuminate the yellow engine light also know as the Check Engine (CEL)or Service Engine Soon (SES) light.

Depending on the year, make and model of your vehicle, this leak could be as small as .020” or twenty thousands of an inch ( a hole equal the point of a sharpened pencil lead)


Now with any diagnosis, the first thing we what to do after verifying the concern is to check the codes. This can be done at just about any chain auto parts store or if you own a code scanner, you can do it yourself.

DO NOT CLEAR THE CODE!!!!!!!!!!!!!!!!!!!! This is your key to finding out what is going on……we will clear the code after we repair the vehicle.


EVAP codes are considered either TYPE A or TYPE B codes..which in simple terms means the control module must see 1 failure for a TYPE A or 2 or more back to back failures for a TYPE B before the SES or CEL light is turned on.

Lets take a look at the codes and what they mean

P0440 or a P0455 – This is a Large leak indication or as people call it “The GAS CAP code”

This is a TYPE B code. Usually the code is caused by the gas cap being left off or loose after a refuel. This code could also be caused by any failure of the EVAP system that will cause a large leak.

P0442 – This is a Small leak indication.

This is a TYPE A code. This indicates a leak as small as .020”.

This code needs some serious diagnosis and knowledge of the system to find and repair the leak. Usually specialized equipment will be used to identify the leak.


P0446 – This indicates there is a restriction or blockage of the Vent Solenoid.

This is a TYPE B code. Any thing that prevents the venting of the EVAP system will set this code…usually dust and debris in the Vent solenoid or a damaged vent hose will cause this.


The other EVAP codes (P0443,449 etc) are electrical codes related to various components of the EVAP system.

Electrical codes need some good diagnostic skills, knowledge of the system and some specialized equipment.


With any car repair concern, there is a logical flow to follow when trying to identify what is going on..here in the GM world, we call it Strategy Based Diagnostics or 7 simple steps to diagnose a concern.

As always, verifying the concern and then doing a thorough visual inspection is key. It’s amazing how quickly a problem can be fixed by just the results of a visual inspection.

Now getting back to EVAP concerns….once you have identified the problem, you must come up with a repair, then a verification that the vehicle is repaired properly.

With EVAP leaks, usually a large leak is caused by a loose gas cap. Reinstalling the cap and clearing the code will be a quick remedy. But remember that you must always verify your repair..this could include professional intervention…(EEK!!!!!!!!!PAYING SOMEONE TO WORK ON MY CAR…..NO WAY!) . Well yes, this may be necessary..I have seen many times where people have come in to have a EVAP code cleared and refuse diagnosis only to have the light come back on…then blame me for the light coming back on…(then post on AF how the “stealership” did not fix their car).

But this isn’t a rant, this is information on what is going on.

Repair verification could include EVAP system testing with specialized equipment to verify that there are no leaks present..I personally check every EVAP system for leaks even if I find the customer left the gas cap loose…this insures that there are no other problems with the system that would cause the customer a trip back to see me. Far too many times have I seen someone tighten a gas cap and clear a code only to have the vehicle come back a few days later with a “real” EVAP leak…masked by a loose gas cap.

If you have a SES or CEL light caused by an EVAP concern and you have the code cleared and it comes back on after a few days….get to your local repair shop ( I recommend your local dealer with factory trained technicians) and have the system tested and repaired properly.

If you want to DIY the repairs, feel free, there are plenty of resources on the internet, but just don’t jump in and replace parts…educate yourself on the system and perform some diagnosis…many times a part will not fix the concern…

Good Luck






if someone wants to take that info and pin it somewhere i bet alot of people have herd of this problem or had it.

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yes.. i hate j-lube but i did not have much of a choice... i should have told them to go ahead and do the front dif and t-case. that would have been funny.


Don't laugh... they probably would have charged you for it! :shakehead:

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Don't laugh... they probably would have charged you for it! :shakehead:



i think if they would have charged me i would have had to call them out on it...


Oh, I'm sure. I probably should have said... "they probably would have tried to charge you for it! "

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lol but anyways. just thought i would let every know we went out to get the code. and had the truck running and before he could open the program on his laptop the CEL went out. so the gas cap fixed it.


and thanks for everyones help.



and i hope that info i posted helps someone out.

Edited by red04supersport (see edit history)
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Have you checked your oil level since Iffy Lube changed the oil?


Make sure they put 6 quarts back in.... :yellow_loser:



hahah yes i did, they said my truck was done. i told him not to move my truck. i paid them walked out there checked the oil level, started it then i checked it again. i know someone who left with 0 oil in there car. lets just say j lub bought them a motor.

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Are you 2WD or AWD, you info panel under your Avatar says AWD.......






Secondly, what are the exact codes you threw? There are a number of possibilities that could be the culpret........has the light come back on?

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