We saw a check engine light on our Corolla. We did some Internet research to diagnose the problem and decided it was not a job to try ourselves. So we took the car to VA Tire and Auto in Chantilly. Their diagnosis pretty much confirmed what we thought. The fix wasn't cheap, but they were nice enough to honor a couple of coupons for the diagnosis and repair. The light is gone, and the car is running better and getting somewhat better fuel economy.