Visual and Physical Checks

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.

Check the Powertrain Control Module (PCM) ground connections for being clean, tight and in the correct location.
Check the PCM for the correct CNG calibration. Program the PCM if necessary. See PROGRAMMING in SELF-DIAGNOSTICS - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE - GASOLINE article.
Check the vacuum hoses for leaks, restrictions and proper connection as shown on the Vehicle Emission Control Information (VECI) label.
Check for any type of fuel leak or fuel line restriction.
Check for air leaks at all areas of the intake manifold sealing surfaces.
Check the ignition wires for cracking, hardness, proper routing or carbon tracking.
Check vehicle wiring for proper connection, pinches or cuts.