Disconnecting the battery clears the ECU's internal data. (you can also clear it by pulling the fuse for the ECU.)

One of the sensors is bad or not connected properly (bad connection, short in the wire) to the ECU (or you have a bad engine ground).

Look under the carpet where your front passenger's feet go. That's where the ECU is.

When the check engine light is on, the ECU's LED will flash.

Count the flashs that will give you the error code. (there can be more than one code displayed at a time; flash-flash-flash, pause, flash-flash-flash-flash-flash, pause, repeat = code 3 and code 5).

Lookup the code in your repair manual and work from there.