I have a 02 silverodo battery went dead i charged battery reinstalled it it starts but dies and with the key out of the ignition security light stays on

The alternator cold be bad and will have to be replaced.

So what does your mechanic think is wrong ? Battery dies overnight while sitting or when driving the battery goes dead ? Two different problems . Going dead overnight means there is a parasitic drain on the battery . ( something is staying on ) Going dead while driving means the alternator isn't charging the battery .

The low or dying battery may have tricked the alarm to set off. Have your battery tested. It will have to be charged up again first, but it should be fully tested. Either it or your charging system may have a problem to cause a dead battery in the first place.

When you have reinstalled a good battery, turn the key to on and observe the security light and the check engine light. They should both come on as a bulb alert that both are working, and the security light should turn off in about 5 seconds. The check engine light should go off as soon as the engine is started. If all systems go, so far, then now would be a good time to do a simple test of the charging system. Anyone with a voltmeter could do it.

Try the reset for the key switch , hook up battery charger or maintainer to battery then turn key on do not crank engine leave key in on position for ten minutes , repeat a total of three times then turn key switch to off position after third cycle then remove key . Then reinstall key to ignition switch then crank the engine over and see if that fixes your issue .

Likely a bad fuel pump. However, if the security light is flashing, leave the key on until the security light stops flashing. Ten minutes with the key on. When the light stops flashing and stays on all the time, turn off the key. Now, start the car. You may be surpised. If your battery went dead and you charged it up, the security prevents you from stealing your own car! Have fun.

the key lost synchronization with the the computer. its a common problem with these new high security systems they're very sensitive. this made the battery die, next step is to take the car to the dealer, dont try to start the car anymore. if you have any questions call Priority Automotive Locksmith 866 999 7858

have you had the alt checked.If ok have a load test done on battery it might have a bad cell

You will have to get the computer reprogrammed at the dealer.

WELL DID YOU REPLACE BATTERY IF YOU DID OR IF BATTERY WENT LOW IT MAY HAVE SET SECURITY SYSTEM ANY WAY IF YOU HAVE SECOND KEY TRY IT FIRST IF NOT PUT KEY IN RUN POSISION FOR FIFTEEN MIN OR SO SEE IF LIGHT GOSE OUT THEN TRY TO START CAR IF YOU HAVE TOW KEY TRY THIS WITH BOTH IT COULD BE THE CHIP IN KEY WENT BAD BUT IF THIS DOSE NOT WORK THE WIRE SOME TIME BRAKES AT STEERING COLUM SO YOU HAVE TO FIX VERY COMEN HOPE THIS HELPS DO NOT LET BATTERY DIE WHEN YOU DO THIS SO IF YOU HAVE BATTERY CHARGER PUT ON WHILE YOU TRY THIS OR MAKE SHERE BATTERY IS CHARGED

not really charge battery n try again