2002 avalanche 5.3. turn the key and it starts, but only for a second and then shuts down, any suggestions. problem just started and havent tried any rpair

I, feel sorry for you, after reading your problem, but don't worry you visit the Reliable-Store they will help you for sure.
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GM has had problems with their antitheft/security systems for about 30 years. The problem is that when the system can't read the key properly, the computer shuts down the fuel injectors after 2-5 seconds of starting the vehicle. This to prevent HOTWIRING the vehicle, but tends to hurt owners when it fails more often.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

Check the ignition coil or ignition module something is not getting a spake

I would suggest checking/replacing coil packs, and fuel pump operation. If they are good maybe a bad ignition switch is possible.

YOU MAY HAVE A BAD DOOR AJAR SWITCH. HOWEVER, YO HAVE A TIMER THAT SHUTS OFF THE INTERIOR LIGHTS AFTER 300 MINUTES. IT'S CALLED A BATTERY SAVER FEATURE. TEST IT OUT.

THE EXTENDED CRANK MAY BE A CHECK VALVE IN THE FUEL PUMP IN THE TANK, ALLOWING PRESSURE TO DROP WHEN YOU TURN THE ENGINE OFF.
OR A BAD FUEL PRESSURE REGULATOR ON THE ENGIEN FUEL RAIL.

TRY THIS TEST, TURN KEY TO "ON" POSITION, WAIT 5 SECONDS, SHUT OFF KEY FOR 30 SECONDS. TURN KEY ON, WAIT 5 SECONDS, THEN START ENGINE. IF IT STARTS RIGHT UP, YOU ARE LOSING FUEL PRESSURE WITH ENGINE OFF.

THIS BUILDS UP PRESSURE IF IT DROPPED DURING SHUT DOWN.
REPORT TEST RESULTS.

PLEAE RATE, THANKS

Turn on the ignition with engine OFF.
Try to start engine, then release the key to ON
Wait 10 or more minutes till SECURITY TELLTALE turns OFF.
Turn OFF the ignition, wait 5 seconds.
Repeat this procedure 3 more times, it takes 30 minutes.
You must turn the ignition key OFF before starting the engine.
To be on the safe side OFF means take the key off, and hold on your hand for 5 seconds.
If you missed something visit GM dealer, and they gladly will provide you a print out of this procedure.

YOUR CHECK VALVE IN THE PUMP IS ALLOWING THE FUEL TO DRAIN BACK ONLY FIX IS TO REPLACE PUMP.

my eclipse did that. It was the fuel pump. It finally just died one day and wouldn't start back so i would go ahead and replace it.

Sounds like your security system is shutting the car down. You should be able to leave the key on for 15 minutes and it will reset itself. Make sure you are using factory keys, and they are in good condition.

It sounds like a fuel pump problem. When you first turn the key on you should hear the fuel pump run for about 5 seconds then shut off. There is a fitting on the fuel rail on the engine that looks like a tire valve, try pushing on that to see how much pressure there is after the pump runs. There should be a good amount of pressure on it. If fuel just dribbles out or there doesn't seem to be much pressure it is definaly a fuel pump. The best thing is to use a fuel pressure guage that will attach at that fitting. There should be between 45 - 60 PSI of fuel pressure when the pump is running. Good luck with your problem.