Very interested to see what the dealer says on this. I also have a 2020 with 1300 miles and on my last full tank from sams club 91 octane the car was running rough. Check engine light did not come on but the full tank only lasted 200 miles. Mpg on the dash was marking 7.9 mpg at most. This was 90% hwy driving average of 70. Dealer said to bring it in when the check engine light comes on. The following tank from the same sams club with 91 octane the car returned to normal. I chalked it up to just a bad batch of gas.

So of course when I got in the vehicle this morning to leave for the dealership, the lights weren't on anymore and everything is normal. I still chose to bring it to the dealer, in hopes they could see the history of the codes. It seems more and more like it will be a gas related issue. I'll report back once they tell me what they find.

Thanks again for the help and suggestions.

It is like going to the dentist....

Good job

I'm betting on a "could not replicate/duplicate" response from dealer along with some advice to never add more fuel after the pump automatically shuts off and to tighten the gas cap per the owner's manual.
I would bet they don't even test drive it except in and out of the garage, especially if there is not current or historical codes.

Okay, here's the deal.

The dealer is able to see any codes that have been present on the car. They said the code was thrown for "rough idling" yesterday. They don't really know why though. The technician attempted to get it to throw the code again, but it never came on. The service manager said they called it into Lexus Corporate to ask if any other GX have had this issue. He said if they hear back from them, they will call me. He also said if corporate recommends something they will have me bring it back in.

I'm beginning to believe that it's all gas related. The car was purchased 4/20. It now has a little over 500 miles. It literally sits in my garage for weeks at time without ever moving or even turning it on. Maybe gas can be "old" or "bad" if it sits too long. The other contributing factor was me filling the tank on Saturday while also topping it off with an additional 1/2 gallon after the pump clicked off. As hot as it is, that was likely not smart. However, I did run some errands and drove it over 10 miles after the fill up, so there should have been room for expansion. Also, once I got to the golf course yesterday morning, the first thing that I did was check to make sure the cap was completely tightened. When I took that cap off, there was a LOT of gas vapor that escaped. Was that the issue? No way for me to really know.

Everything seems fine now, but I'll definitely keep an eye on it. Also, I will never top off the gas again.

Should be find going forward... if it gets ya to the golf course (fellow golfer here) that's really all that matters ;-) Wouldn't hurt to drive it more often... we drive ours once a week whether we need to or not, going back to the ole weekend drives the past 4 months here...

Consider the issue a breakfast ball, mulligan on the first tee of ownership.
FORE!

Haha! Fore, indeed.

Thanks everyone!

Kinda-sorta. Gas will get "bad" after a year or so but that usually applies to the stuff in your lawn mower at the beginning of the season or that 2 gallon can you have in your garage.

However, buying gas in a little frequented station isn't good either as who knows how long that gas has been there.

I think you're OK
now - just drive a bit more.

I had the same thing happen to my 2020 GX with 10K miles. Check engine light on with traction control, etc and a very rough idle. Light went off on the way to the dealer and they could not replicate it. The code was for a MAF sensor, but said that they can't replace if there is not an active code. It's been fine for last few weeks with no issues.

Interesting, anyone check on their secondary pump and get a list of codes?

These are typical SAIP codes

P2440 - Secondary air injection (AIR) switching valve, bank 1 – valve stuck open
P2441 - Secondary Air Injection System Switching Valve Stuck Closed Bank
P2442 - Secondary Air Injection System Switching Valve Stuck Open Bank 2
P2443 - Secondary air injection (AIR) switching valve, bank 2 – valve stuck closed
P2444 - Secondary air injection (AIR) pump, bank 1 – pump stuck on
P2445 - Secondary air injection (AIR) pump, bank 1 – pump stuck off
P2447 - Secondary air injection (AIR) pump, bank 2 – pump stuck on
P2448 - Secondary air injection (AIR) pump, bank 2 – pump stuck off

It may make no difference, but try removing then resecuring the gas cap. My Lexus does something similar in winter aaaall the time when it's particularly cold out (less than 40°). I've also noticed it has a propensity to do this less when I fill-up, too.

This is exactly what happened on my 2008 GX, rough idle and all. Took it to dealer, pulled into service, host resealed the gas cap which immediately fixed the problem. Still a good idea to take it in, especially if it's under warranty, but I agree that the gas cap is 99.9999% the problem.

With mine, it was the VSC OFF, VSC TRAC, Anti Skid, and Check Engine lights all coming on simultaneously. A new fuel cap fixed it...for a year. The following year, winter returned and so did my lovely Christmas tree of errors on my instrument cluster. Replaced the fuel cap again, the problem went away again...for another year. This time, I just turned a blind eye. Spring came and the problem went. I think it's just a subpar gasket they're using for the caps. And since I use 100% 93, I know it's not anything to do with ethanol eating away at the rubber gasket. Still, no idea why I'm good for a year at a time.

I had a similar check engine thing with our 2007 GX. After taking it to the dealer, it was a bad Gas Cap. The gas tank runs under vacuum and there is a valve in the Cap that goes bad. I replaced the cap twice in 12 years.