DTC P1204: Vehicle Does Not Operate In CNG Mode: Description
WARNING: This page is about a different car, the 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The Powertrain Control Module (PCM) controls fuel delivery and determines which fuel system the engine is operating on. The PCM controls the Low Pressure Lock-Off (LPL) solenoid, and the High Pressure Lock-Off (HPL) solenoid. The PCM commands the HPL solenoid open for one second at every ignition ON, to prime the CNG fuel system. The PCM commands the HPL, and the LPL open, when the engine is cranking or running on CNG. If the vehicle does not start and operate on CNG within 8 seconds, this DTC will set.