You need to scan with an OBDII tool to find the stored code(s) that tripped the SES light.
A battery disconnect is essentially a 'reset' that clears the code(s), and after you've driven/run through a series of cycles
[see
http://www.obdii.com/drivecycle.html], you'll give the ECU enough time to gather all the data it uses to assess if the SES should be illuminated.
You can find out more about OBDII at
http://www.obdii.com/
My '96 3-cyl. will throw a P0420 every so often, but it burns cleanly enough and gets decent mpg (over 40).