Happy Friday Eve everyone!
Just bought a 2007 is350 with 170k miles. Runs smooth like butter, very quiet and smooth idle.

A couple days ago I tested the 0-60 in sport mode and after I let off the gas at 60mph the car stayed in the gear and would not upshift. I got worried so I switched the mode back to normal and proceeded to drive and it shifted up a gear and everything was fine.

When I got home I noticed the CEL on(as well as the VSC light). Everything seemed fine...smooth idle etc. But I noticed the idle was lower than normal, it was almost at 500rpm whereas its usually around 650rpm. And about every 3-5 seconds I could feel a sort of shudder in the idle for a split second. I could also hear it in the exhaust, like an uneasy idle but the tach never indicated the idle was changing.

I drove it for a couple days and everything seemed fine, and I didnt get a chance to go by AutoZone to get the code read. Tonight after I ran some errands, and the weather got much colder than it has been, no more CEL and the idle is back to 650rpm. And I am still getting that shudder which I don't remember having before...but I could be wrong.

Any ideas what may have caused the CEL to come on for a few days, and then possibly go away because of much cooler weather?

I read about the gas pressure sensor, so I ruled that out by filling the tank.

I also smell a lot of exhaust if the windows are down and I accelerate quickly, so I think there is a leak near the header. I don't smell anything when the windows are up, so i don't think it is a leak further back.