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.
Circuit Description
The Alternative Fuels Engine Control Module (AF ECM) and Powertrain Control Module (PCM) are connected by Alternative Fuels Operation (AFO) signal circuit. In order for engine to start and operate on CNG, 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 signal circuit when CNG operation is desired. Bi-fuel (KL6) equipped vehicles primary fuel source is CNG. The vehicle will only switch-over to gasoline operation if one of the following conditions is present:
- Fuel Tank Pressure (FTP) sensor indicates CNG tank has less than 200 psi (1379 kPa).
- Fuel Rail Pressure (FRP) sensor indicates low CNG fuel rail pressure.
- A system fault is detected by AF ECM.
- Every 100th start, vehicle will start and run briefly on gasoline. After this, a switch-over to CNG will occur during first decel condition.
- Engine cranks for 8 seconds and fails to start on CNG.
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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.