If your 2003 Ford F-150 is in theft mode, it means the truck's anti-theft system has been activated and you'll need to deactivate it before you can start the engine. The good news is, there are a few different ways to do this.
One way is to use your truck's key fob. If you have a key fob with a panic button, simply press and hold that button for about three seconds. This should deactivate the theft mode and allow you to start the engine.
Another way to get your Ford F-150 out of theft mode is by using the manual key. First, insert the key into the ignition and turn it to the On position without starting the engine. Next, press and release the gas pedal three times within five seconds. Finally, turn off the ignition and remove the key. This should reset the anti-theft system and allow you to start the engine normally.
If neither of these methods work, don't worry - there's one more thing you can try. Once again, insert your key into the ignition and turn it to On. this time though, leave it in that position for at least 15 minutes without starting the engine or moving any other controls (like turning on lights or opening doors). After 15 minutes have passed, turn off ignition and try starting your truck as normal - hopefully this will do trick!