My 2003 Tahoe runs great, but the security light on the instrument panel stays lit when the truck is running. It does shut off when the key is off and doors are locked. What's the problem??

Anti-theft system problem ? Is there a SECURITY light on the instrument cluster that stay's lit ?

Anti-theft system problem , possibility ! Do you have a security light blinking ? Security Indicator
The security indicator is located in the clock on the top of the instrument panel. The light emitting diode (LED) displays the immobilizer system status. The immobilizer contains the LED driver hardware for direct connection of the LED.
Vehicle Theft Deterrent (VTD) Operation
The immobilizer system design prevents vehicle theft by interrupting the engine operation. The immobilizer system prevents the vehicle from starting. The engine operating functions interrupt if the immobilizer system does not recognize the transponder identification code of the ignition key. The immobilizer system and the content theft deterrent system do not interact. The system includes the following components:
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The ignition key

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The column-mounted theft deterrent module with integrated antenna

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The transponder

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The engine control module (ECM

Does the check engine light come on when you turn the key to run position ? not start , run . Is there a security light lit .padlock an car symbol .

Is the security light lit ? I can't make out what your trying to say here ? Full power ? I take it to mean the battery is good an fully charged ! Do the headlamps work an do there shine bright ? They save or is it they say passcode anti-theft system problem , if so who said ? If the security light isn't lit on the instrument cluster it's not a passlock system problem . If it is lit your best course of action is take it to a GM dealer ship .

Is there a SECURITY light lit on the instrument cluster ? What's the fuel pressure at ? Do you know how to test fuel pressure ? Does the check engine light come on when turning the key to run ,not start just the run position ? Did you try holding the gas pedal down a little way ? Go to you tube an check out videos on trouble shooting , engine won't keep running .

How long does it run before shutting off ? Do you have a SECURITY light lit on the instrument cluster . How many miles on the vehicle , has it been well maintained ,oil , filters ,tune -up etc... Check fuel pressure an flow , if not familiar with this there are a lot of videos on you tube . Will it keep running if you hold down the gas pedal a little ?

The car will not stay running ?How long ? dies right away ,runs for a bit , till warm ? Any lights on the instrument cluster lit or flashing ? Security ?

you may have a bad key check to see if there is a light lit up on the inst cluster showing a key or security if so the key is no good and it will have to be brought to the dealer to repair this problem.

Check your security light on the dash and see if it stays on or blinks with the key in on position. If so google how to reset the security.

Not sure of the year of your Tahoe but I can tell you the standard procedure to program your key for say like a 1999. First make sure your battery has a full charge. Next if you have any security related trouble codes stored in your computer, you will need to clear the codes either by clearing them with a scanner or disconnecting the battery for at least 10 minutes to clear the codes. The first step is to turn the key and just let it turn over quick but not enough to where the vehicle starts and shuts down again. Just kind of tap the key as if starting it so the starter turns the engine for a second. When you do that just release the key so it stays in the "Run" position. Lights will be on in the instrument cluster including the security light. The security light may flash on and off. Leave the key in the run position for 10 minutes. After 10 minutes the security light usually goes out but not in all cases. After 10 minutes in the run position, turn the key to the off position but do not remove the key. Wait 10 seconds and do the first step again (Tap again let key stay in run position for 10 minutes). You will need to do this 3 times totally about 30 minutes. On the 3rd time when the 10 minutes is up, turn off key, do not remove the key and then start the vehicle. This procedure should work to progam your key. If it doesn\'t you may have another problem with the security system. Let me know how it turns out and I hope there isn\'t another problem and the computer just lost the transponder code. there is a security relay/module that sometimes goes bad in those Tahoes but try programming the keys first. If that doesn\'t work and you need further help as far as that relay/module & to let me know how it turned out,