Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 220 (ENGINE CONTROLS SELF-DIAGNOSTICS - 2.2L BI-FUEL)DIAGNOSTIC TESTSDTC P1204: VEHICLE DOES NOT OPERATE IN CNG MODEDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
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.
- Did you perform the Diagnostic System Check - Engine Controls (Alternative Fuels). If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS (ALTERNATIVE FUELS) under SELF-DIAGNOSTIC SYSTEM.
- Observe the DTC information using a scan tool. Are there any DTCs set? If yes, go to next step. If no, go to step 5.
- Using a scan tool, select CAPTURED INFO to store the PCM DTC information. When action is completed, go to next step.
- Turn ignition switch to RUN position, engine OFF. Observe the DTC information using a scan tool. Does the scan tool display DTC P0005, P0191, P1209, P2146 or P2665? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
- Attempt to start the engine. Does the engine start and idle? If yes, go to next step. If no, go to step 7.
- Observe the Freeze Frame and/or the Failure records data for this DTC. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING THE DTC . You may also operate the vehicle within the conditions that you observed from the Freeze Frame and/or the Failure records data. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENTS in TROUBLE SHOOTING - 2.2L CAVALIER - BI-FUEL - CNG/GASOLINE article.
- Turn ignition switch to RUN position, engine OFF. Observe the Fuel Rail Pressure (FRP) sensor parameter using a scan tool. Does the FRP display remain steady near 45 psi (310 kPa)? If yes, go to next step. If no, go to FUEL SYSTEM PRESSURE RELEASE (CNG) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE article.
- Verify that there is good spark to the spark plugs. If an ignition condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 13. If no, go to next step.
- Turn ignition switch to OFF position. Disconnect the FICM harness connector. Probe the injector monitor circuit using a test light connected to a good ground. Turn ignition switch to RUN position, engine OFF. Does the test light illuminate? If yes, go to next step. If no, go to step 11.
- Check for an intermittent and for a poor connection at the FICM. Did you find and correct the condition? If yes, go to step 13. If no, go to step 12.
- Repair the open or short to ground on the injector monitor circuit. After repair, go to step 13.
- Replace the FICM. After repair, go to next step.
- Clear the DTCs using a scan tool. Turn ignition switch to OFF position for 30 seconds. Turn ignition switch to RUN position, engine OFF. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING THE DTC . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step 2. If no, go to next step.
- Observe the CAPTURE INFO using a scan tool. Are there any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.