My 2002 did the same. Usually if all else is fine, it's the spark plugs or worse the coils. The engine probably misfires causing the computer to let you know there is an emission problem that could affect your cat's. Mine was misfiring cyl. 3,5,8. Start with the spark plugs.
If the engine light flashes that means that the comp. senses a condition that could damage the cat. convertors if left unchecked which is very costly. Go to autozone and get a free code reading as soon as u can to avoid this. Then you will have a better idea what is going on and if u can attempt to fix yourself or not.
Mine did this and when read on the computer it said #3 cylinder. It was the coil pac on the #3 cylinder. The test at oreillys would only tell me #3misfire nothing else had to take it where they had a different computer that would give more information. It cost me 192 dollars for computer reading, coil pac and labor. It fixed the problem though.