Just bought a used 1998 Honda Accord - check engine light is on? Ped smog fine!?

Hi, just bought a used Honda Accord. They did a smog test prior to sale ( I have the receipt and certificate of ping), new timing belt about 10,000 miles ago, new brakes.. well taken care of. What in the world could this be? I hear its often the cat, but I'd be surprised based on the fact it just ped smog days ago. Any ideas?