1997 Lexus ES 300 I made a mistake.

Yup; trade 'er in for a Dodge!

Actually, even though the engineers are insane, they did take into account this could happen, either accidentally or on purpose. This is what is causing so much trouble this time for GM. On older cars with carburetors, fuel would keep going into the engine as long as it was still spinning, then if you turned the ignition switch back on, all that raw fuel in the exhaust system would explode and it could blow up the muffler. Your car has electronic fuel injection, and that got turned off so nothing serious is going to happen. The engine Computer detected some problem that could adversely affect emissions so it set a diagnostic fault code and turned on the Check Engine light to tell you. If the code doesn't erase on its own and the light goes out after a few days, have the code read at an auto parts store. Most will do that for you for free, and they can erase it.

What you did is what a lot of driving instructors do so students see that the car can still be steered and stopped, but with significantly greater effort than normal. Any car can develop an intermittent stalling problem, and there are still some drivers who think they can't steer or stop. They panic and crash, then scream, "lawsuit". In fact, everyone should know how to coast safely to a stop with a dead engine. It's really no big deal.

It depends on the seriousness of the problem the computer thought it detected. There's well over 2,000 potential fault codes. Only about half of them refer to things that could adversely affect emissions, and those are the codes that must turn the light on. If the problem is intermittent and goes away while you're driving, the light will go off too while you're driving. If it's a little more serious, the light will stay on, even if the problem goes away, and will go off the next time you turn the ignition switch on. If it's still more serious, the light will "latch" on and always be on when the ignition switch is on, even though the problem went away.

The most serious problems are when the Check Engine light is flashing. You're supposed to stop the engine right away. Too much raw fuel is going into the exhaust system and it will overheat and damage the expensive catalytic converter.

Sensors don't get reset. They just do their thing and report things to the computer. The problem is no two sensors are ever exactly alike. The computer learns the characteristics of each one by comparing their readings and engine operating conditions to each other. When you disconnect the battery, that will erase most fault codes, but it erases all the other stuff in all the other computers' memories. Your car is new enough to have at least two dozen computers, and most of them have memories that will get erased.

Your Engine Computer also learns "short-term fuel trims", (STFT), and "long-term fuel trims", (LTFT) as you drive. Those "look-up tables" will all have to be rebuilt the next time you drive the car after the battery was disconnected. You likely won't even know that is taking place, but until that is completed, fuel mileage, emissions, and engine performance won't be at its best.

Most importantly, too many people obsess over doing whatever it takes to get the Check Engine light to go off. You have an unusual case where we know what caused a problem to be detected, but under normal conditions, when the Check Engine light turns on, the last thing you want to do is disconnect the battery or let it run dead. The computer set the fault code to provide really valuable information, and you don't want to lose that until the codes have been read and recorded.

Think of that fault code as going to the doctor with a pain, and you can tell him where it hurts. Erasing fault codes, then trying to figure out where to start troubleshooting is like taking your dog to the vet. He can't explain where it hurts.

Remember, about half of the codes don't turn the Check Engine light on, so you could be driving with a problem the computer detected and never even know it.

The codes don't refer to expensive things. They refer to a defect in a circuit or system that may or may not be caused by the part when one is referenced in a code, or the unacceptable operating condition. Where things have the biggest potential of turning expensive is when the Check Engine light is ignored. Even if the codes are read and recorded, and it's assumed the problem isn't serious, if you drive with the light on, how will you know if a different problem is detected that might be more serious? In your case we know there is no defective circuit because you can't cause a part or a wire to fail by turning the engine off. The computer had to detect some operating condition that it didn't like.

At the first chance you get, just drive to an auto parts store and ask them to read the code and erase it. Don't pay too much attention to the description because they're just using simple code readers, and their descriptions of codes can be confusing or misleading. It's the exact code number we want to see, then we can look it up here to see what might have happened;

https://www.2carpros.com/trouble_codes/obd2/P0100

You can blame that on your nut-job politicians. They forget about the "law of unintended consequences". What is the harm in finding out why your Engine Computer is unhappy, and perhaps repairing that? You gotta wonder how many people will put off getting their polluting vehicle fixed right away, and how long they'll drive it like that if the law says they have to buy someone's service that they might not be able to afford.

Because of the laws you're bound by, my recommendation for the next time the light turns on is to buy your own code reader. Some of the better ones cost less than a hundred dollars. I believe visiting a dealer or an independent repair shop can be a good value, but there are times when it is appropriate to do some diagnosis and repair on your own, depending on your skill level, ... And of course when you have geniuses like me to walk you through it! Saving the cost of one hour of the mechanic's diagnostic time will pay for the code reader.

If you really want to have fun you can buy a regular scanner too but that is a really big investment. I have a Chrysler DRB3 scanner. A lot of independent shops bought them because along with doing everything on Chrysler products and accessing every computer, with an additional plug-in card they will do emissions-related stuff on all brands of cars sold in the U.S. Starting with '96 models. That includes reading and erasing fault codes. You can find these on eBay. I bought mine through the dealer I used to work for. You might consider charging friends and coworkers a few bucks to read their fault codes, (their cars' fault codes), to pay for the scanner. If that makes the politicians upset, so much the better!