High Pressure Lock-Off Solenoid
WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The HPL is a normally closed, solenoid valve. The HPL solenoid and a Low Pressure Lock-Off (LPL) solenoid in the fuel line prevent fuel flow. The Powertrain Control Module (PCM) commands ONLY the HPL open for one second at EVERY ignition ON to prime the CNG fuel system. The CNG prime pulse charges the fuel line which allows the Fuel Tank Pressure (FTP) sensor to monitor the amount of fuel pressure in the system. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates that the engine is cranking or running on CNG.