Antitheft is flashing fast. Car wont start.

Hello, The PCM and key are two of four items that must agree. The antitheft module on the steering column, key, PCM, and instrument cluster (which acts as a control module) must all be coded to the same vehicle, and the key must match the antitheft module (and maybe the instrument cluster). There is a chance that flashing the PCM did not work as intended, but you will certainly need diagnostic scanning to identify where the security lockout is being triggered. You can scan for codes to see if there is a message stored on the antitheft or body control module.