'96-and-up Mustangs have a unique security system called the Passive Anti-Theft System (PATS), which can cause problems for DIY engine swappers who aren't familiar with it. PATS is comprised of a steering column-mounted transmitter and an ignition key with a coded chip (arrow) that all work together to protect the 'Stang. When you attempt to start the engine, the PATS transmitter sends out a radio frequency signal that's picked up by the transponder in the key. Once the key's coded chip is recognized, the transponder returns a unique RF signal to the PCM, giving the OK for the engine to start and run. Engines swapped from PATS-equipped 'Stangs have no hope unless the processor, coded key, and transponder package are part of the process, or the PATS system is disabled with a plug-in chip or flash tuner.