My check engine light came on four days ago. The car still runs well. The light comes on every time I start and drive it for more than 3 minutes. Also where is the diag port if there is one. Bought the car used and its my first Honda.

From what I understand most cars are programmed for the light to come on when it hits certain mileage points whether it needs anything or not. You have to take it to a garage to get it reset but it is most likely a good idea to get whatever routine maintenance it might have not gotten within the time.

Most OBDII (On Board Diagnostics) ports are easily spotted and located just under the driver's side dash.

AutoZone will check codes for free. Sounds like it may be for an oxygen sensor, after it comes out of "open loop" mode.