I replaced my car battery yesterday, and now my check engine light is on. When I connect the ODB2 code reader, it says no codes found. Can anyone explain what is going on and how I can fix it please.

 

Drive it. If there were no codes, it may turn itself off. If not, check for codes again in a few days. If still no codes, try this. Disconnect the battery and leave it disconnected for an hour or two (be sure to have the radio code just in case). Reconnect the battery and turn the ignition on but DO NOT try to crank the car. Let the ignition sit in the On position for about 15-30 seconds. THEN proceed to start the car. This should reset the ECU and clear any junk data left over in the memory. The engine may stumble at first but should run fine after a minute or two.