The light is for the catalytic converter that is now missing. It's a p0420 code. If you turn the light off, it'll pop back on. You need an O2 simulator. The easiest way is to get an 18mm long spark plug anti-fouler (auto zone $4), drill it out so you can screw the SECOND O2 sensor into it, then screw both back into the exhaust, the light will never come back on.

To check codes: Turn ignition on, do not crank car, count to 3, then press the gas pedal 5 times, after the last press and let off is finished, count to 7, then press and hold the pedal for 10 seconds, the check engine light should start to flash. Read the codes as follows, 10 flashes = 0, 4 = 4, 5 = 5, ect. you should see 10 flashes, then 4, then 2, then 10, thats 0 4 2 0. If no other codes are present press and hold the pedal for at least 10 secs, release, then shut the ignition off, the code should be gone. Make sure you count slowly or use a watch untill you get the hang of it.