The fuel system has a set of injectors. These injectors have a solenoid, which is energized, also raises a valve inside the injector to distribute fuel in the engine. Generally, one of the two injector terminals has battery power. When an injection is requested, the PCM (Powertrain Control Module) controller connect to ground the other terminal. This supplies the injector circuit.
Diagnostic code P0203 OBDII is established when the PCM has detected a malfunction in fuel injector circuit number 3. This malfunction is due to the PCM ordering the injector to activate or deactivate, but not observing the corresponding change in voltage.