Anti theft light came on,stayed on for a few days.The light fanally shut off,now the truck starts for a couple seconds then stalls out.How can i fix this problem??

Ford anti-theft systems don't act like that . If the PATS anti-theft system were the problem , it would not start at all . GM vehicles will act like that ,start then stall after a couple seconds . Now you will need to have new key's programmed into the vehicle . Depending on the type pats system it can disable the starter , That means the starter motor will not crank the engine . Or it disables the fuel system .

https://www.youtube.com/watch?v=hLX1RR5w5F0

How to Disable a Taurus Factory Anti-Theft System

1. Press the "unlock" button on the remote entry transmitter.
2. Insert the coded key into the ignition. Turn the ignition on. The anti-theft light will turn off after approximately three seconds, indicating the system is disabled. Repeat the steps if the light flashes rapidly or stays on for more than a few seconds.

https://itstillruns.com/disable-taurus-factory-antitheft-system-8452652.html

Anti-theft system has nothing to do with it . Battery cables , maybe , Park neutral safety switch possibly ! With anti-theft system problem you wouldn't a click or anything , that is if the anti-theft disables fuel or the starter . Plus the SECURITY light would be on if it were a anti-theft system problem . Your best bet , take it to a qualified repair shop that knows how to do starter circuit testing >
Starter Voltage Drop
How to voltage drop test starter motor circuit

And what's this identity anti theft system ? No such thing ! The vehicle has a content theft an vehicle theft deterrent system !
This system disables fuel not cranking of the engine. The VCM/PCM will not disable the fuel due to any BCM message for the remainder of the ignition cycle.
If the VCM/PCM does not receive the same password from the BCM as the last learned one, the vehicle will start and then stalls due to the Fuel Lockout.

Sorry to hear about this problem. Unfortunately this is a classic GM antitheft system failure. Believe it or not, this is very common and can be very expensive to take to a mechanic. Do yourself a favor, go to this link and study up in this system. There is alot of information here and you should be able to learn what's going on. There are also some ways to fix this problem yourself at a much cheaper cost.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

Maybe it's the anti-theft system an not the remote start . Is the SECURITY light lit on the instrument cluster ? GM anti-theft systems will disable the fuel injectors if there is a problem .

This could be an anti theft security issue. This happens all the time in GM's. The anti thefts are known to fail and cause the car to not start, stall while driving, or start for 1-5 seconds and then die. A good indication that the security is failing is if you see a light on the dash that says SECURITY or a picture of a car with a padlock thru the window. This link explains the problem, symptoms, and some DO IT YOURSELF solutions. http://vatspasslockpasskeysecurityhelp.yolasite.com/

Fuel filter?

your ignition switch could have carbon on the main contacts you should disassemble it clean it then reinstall it. the after market switches for that car are not very good. the switch is the problem causing the light to stay on. paslock is the reason your car will not run. the easiest why to fix it is- go on the net find a remote start companys one of them will have a genric diagram explaining what they have to do to get around paslock. with this diagram you can see the wire to cut and then insert the proper resister to resignal the paslock then remoce the resister reconnect the wire.

the big brain

I disconnected the + wire from the battery terminal and left it for a couple of days to reset the computer, That's not going to do anything . You never ever disconnect the positive cable untill the negative cable is disconnected first . An disconnecting for 1/2 hour would be enough .
my mechanic told me it was a glitch in the computer, If the check engine light is on there is a DTC - diagnostic trouble code . You need to find yourself a new mechanic .
If nothing happens when you turn the key to the start position U need to find out if anti-theft system can disable the starter circuit .If not check battery condition ,connections an cables doing a starter circuit voltage drop test . Also your vehicle has integrated power module , google that one . There is a starter motor relay in the under hood fuse box , do you know what a relay is ? If you find it , pull it out an look on the bottom , you will see four sets of numbers . 30 &87 .these are the load side (starter ) if you jumped those together with a paper clip or piece of wire . if the starter is good an the cables an connections are good the starter will spin the engine over . Pins 85 & 86 are the control of the relay circuit . 86 should have B+ voltage when trying to start the vehicle , 85 is from the PCM - engine computer . The PCM will ground this circuit internally , if it see's that the transmission is in park or neutral an if thier is no anti-theft system problems .

it sounds like the immobilizer is faulty. this will shut off the electrical feed to the starter motor. you will need to take it to a garage to get this fixed.

hope this helps.