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.
Fuel System Unwanted Switch To Gasoline: Description
Alternate Fuel Engine Control Module (AF ECM) and Powertrain Control Module (PCM) are connected by the Alternative Fuels Operation (AFO) Enable circuit. For engine to switch-over to CNG operation, PCM must be commanded to disable gasoline fuel injectors, switch to a different ignition spark calibration and disable some PCM DTCs. AF ECM performs switch-over command by grounding AFO enable circuit when CNG operation is desired.
This vehicles primary fuel source is compressed natural gas (CNG). Vehicle will only switch-over to gasoline operation if one of the following conditions is present:
- Fuel Tank Pressure (FTP) sensor indicates CNG tank is empty.
- A wide-open Mixture Control Valve (MCV) is detected.
- System fault is detected by control module (AF ECM or PCM).
- Manual shut-off valve is closed.
- Engine crank time exceeds 8 seconds.
- On every 100th start and also when coolant temperature is less than 10°F (-12°C), vehicle will start and run briefly on gasoline. A switch-over to CNG will occur only during a decel condition.
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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.
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