Hello fellow Lexus owners. Found some very good info that might just save everyone some money and that's always good. OK so I bought this 06 GS300(nice ******in car) from this guy and he told me he had a check engine light for the throttle body he replaced and it went away. I took my scanner before I bought the car to check for a recent reset and all monitors were good. I bought the car and took it and it passed the smog check. As soon as I started the car to leave, low and behold I get a check engine light. I check the code and its a P0505 which Idle Air Control, which is located on the throttle body. So i get home give the car a more thorough once over in case I missed something and found everything good. So i went and got some throttle body cleaner and MAF sensor cleaner and cleaned that. I did that throttle reset and everything seemed OK. Drove the car again a day later and seemed good. About 45 min into the drive I notice the CEL was back. The CSC light and check accompanies the CEL(which happens all the time) so i go back home and check the code and its P0505 again. The car feels no different, I felt no surge or anything else i just notice the light when its illuminated. After some research and a couple other ideas I decided OK so maybe this might be something else so Ill take it and get it diagnosed at a Lexus mechanic(not the stealership) and I'll do the maintenance myself. Everyone was closed or closing soon so I decided to look a little more online and see what i could find even though I found everything on the GS and IS and found alternators to ECMs and I'm not about to be throwing parts at problems. Well I come up on a video from a guy with the RX SUV and that had something similar but he had about 7 codes thrown. He had an an emission code and he decided that the gas cap could possiby be the problem with that code so he will eliminate that one and concentrate on the others afterwards. He goes there and the cap looks normal but was easy to unscrew. He pulls it off and feels the rubber o-ring and doesnt notice anything but buys a new gas cap from the dealer anyway. When he compares the two he then notices the rubber on the new one is much more rubbery than he thought and the old one is much harder(how old rubber gets) and the new one has normal resistance to remove. Replaces the cap and he decides he wont reset anything he will let it reset itself. Sure enough after 40 miles no more CEL and ALL 7 codes are gone. So I decided wow, that gas cap got rid of 7 codes, let my try this too. Worse case scenario I bought a $30 gas cap I didn't need, but best case I saved a trip to get a diagnostic and a fee of $50-$100 an hour. I buy the cap and didn't notice much difference but put the new one on anyway. Took the car for a nice long drive and up to this point no more CEL. Then I got to thinking how many of us actually replace a gas cap? this car has over 160,000 miles and how many cycles of cap on/off through its life time? eventually that part wears out but something so small and seemingly irrelevant has a huge impact. So if you guys run into a CEL problem and you cant find out exactly what is wrong, try the OEM(very important) gas cap replacement, because it might just save you a lot of time and money.

 

I saw a receipt for a new gas cap from the previous owner, at a time I think the car had 100,000 km. So yes, they might need to be replaced at one point. However, the code set by a bad gas cap is usually explicitly a fuel circuit pressure code. But if it still solved it, that's fine

 

I was getting P0455 and P0457 periodically.
The OEM gas cap design on GS300 is lousy. No matter how tightly you try to close the cap,
it feels very loose.

I ended up replacing it also few thousands miles ago, and no CEL since.

 

I’ve replaced my gas cap two times in three years because of the check light. After each replacement the light would disappear after a day or so but reappear after a few months. My only theory is there is a design flaw in the cap. Does anyone have any DIY fixes that would make the seal more effective?

 

Im on stock gas cap and rubber since 2008, I never had check engine light issues from it.

 

I think OEM gas cap area has some design problem.
Even when I tighten the cap to 3 clicks, and slight twist (with very moderate pressure)
n the opposite direction opens the cap. It should be more tight and sturdy.

After I replaced it with the aftermarket gas cap from O'Reilly,
I've not had the check engine light for over a year.

I think due to the loose cap design, it's not sitting tight,
and I think when you go over bumps and make sufficient impact,
it just jars it loose, hence causing slight air leak and CEL.

 

Just bought my 06 and was having the same problem with the loose cap. Bought a new one on Amazon...still loose and CEL lights came back on. Bought the Stant replacement cap from O'Reilly yesterday which still feels loose but better than the original and Amazon one. We'll see how it goes during the week.