Disconnecting the battery will turn off the SES light only if there are no hard faults detected by the PCM after re-connecting the battery. It will only clear pending DTC's, not history file DTC's. It will not reset the readiness monitors, misfire history or fuel trim. Most "code readers" can only
read the history file DTC's, that's all. Some of the el cheapo $200 to $400 scan tools like the auto parts stores use, will access freeze frame data, clear the history files and reset the readiness monitors. It requires a high end scan tool, ($2,000 to $10,000 range) to access camshaft retard data, reset fuel trim, misfire history, and perform bidirectional procedures such as crankshaft position sensor relearn, IAC reset, and flash the PCM/VCM etc etc.
Auto trainy:
There are several systems called monitors that run their tests under certain driving conditions. Some run once per drive trip, some once per drive cycle, and some are continuous. Each monitor must run, complete, and pass its test before the vehicle is allowed to be tested for State emission levels. The technician can tell very quickly if all readiness monitors are complete. If they're not, it usually means the system was reset trying to turn off the SES light to cover up a problem. A drive cycle can be completed in about 15 minutes and force all monitors to run and complete their tests. Each monitor has a "set" of requirements that must be completed
exactly to the letter, or the test will not run and/or complete. Under "normal" driving situations, it's possible for a vehicle to go 6 months and not have all of the monitors run and complete their tests. Here is a link to the GM drive cycle:
http://www.obdii.com/drivecycle.html The first time you try it, don't be surprised if it takes you over an hour! It's harder than it sounds.