Possible Causes & Corrections

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Savana, 2003 Chevrolet Silverado, and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

Check the following items:

Check fuel pressure. See FUEL SYSTEM PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Inspect for proper operation of CNG fuel injectors. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
Test the high and low pressure lock-off valves/circuits for proper operation.

NOTE: A higher secondary ignition voltage is required to operate vehicle on CNG than on gasoline.

Check for proper secondary ignition system voltage output using Spark Tester (J-26792).
Inspect for fouled spark plugs. If fouled spark plugs are indicated, repair cause of fouling.
Test for faulty spark plug wires.
Test for an open ignition system ground, if equipped.
Test for proper engine operation by checking for correct engine compression, correct engine valve timing, worn or incorrect camshaft, bent push rods, worn rocker arms, or casting flash in intake or exhaust manifold passages.
Monitoring engine speed on scan tool may indicate EMI is present. A sudden increase in scan tool engine speed while actual engine speed remains constant indicates EMI is present.
EMI on ignition control circuit may cause a missing condition.