It's probably not a big deal. I just had that happen to my 99 ES300. I checked it out and it was the back o2 sensor. That is probably what it is P0420. It is the most common check engine light code to come up on any newer vehicle. I am going to buy an o2 simulator for my car. Replacing the unit which is not important (except for emissions) will be expensive. I love the environment, but the car burns clean. Take your car to Autozone or your local parts store. They will check out the code for free. When you get the code, check out this site to see what it is.
www.obd-codes.com