Document ID# 706188
2001 Chevrolet Cavalier (CNG)
Fuel System Switches to Gasoline

Circuit Description
The AF ECU and the gasoline control module (VCM/PCM or ECM) are connected by the natural gas operation (NGO) Enable circuit. In order for the engine to switch-over to CNG operation the gasoline control module must be commanded to disable the gasoline fuel injectors, switch to a different ignition spark calibration and disable some of the gasoline control module DTCs. The AF ECU performs the switch-over command by grounding the NGO Enable circuit when CNG operation is desired.
This vehicles primary fuel source is compressed natural gas (CNG). The vehicle will only switch-over to gasoline operation if one of the following conditions is present:
• The fuel pressure sensor (FPS) indicates the CNG tank is empty.
• A wide-open mixture control valve (MCV) is detected.
• A system fault is detected by a control module (AFECU, VCM / PCM or ECM).
• The manual shut-off valve is closed.