Practice Test Geeks(CNG) Compressed Natural Gas Practice Test
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System Diagnostics and Repair Flashcards

7 cards from real CNG practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 System Diagnostics and Repair flashcards as text

  1. A CNG vehicle's ECM communication is lost intermittently. The scan tool shows 'No Communication' only when the engine bay is hot. The MOST likely cause is:

    Answer: A heat-sensitive intermittent fault in the CAN bus wiring or ECM power/ground circuit

    Temperature-sensitive 'no communication' faults typically point to wiring insulation breakdown or connector issues that only occur when thermal expansion creates an open circuit.

  2. A Type 4 CNG cylinder (all-composite) shows delamination of the outer shell. The correct action is:

    Answer: Immediately remove from service; composite cylinders with delamination cannot be repaired and must be replaced

    Delamination compromises the structural integrity of composite cylinders; they cannot be repaired and must be permanently removed from service.

  3. When diagnosing a CNG bi-fuel vehicle that runs poorly only on natural gas, the technician should first:

    Answer: Confirm the fault is isolated to CNG mode by switching to gasoline and verifying normal operation

    Confirming the fault is isolated to CNG mode by running on gasoline helps determine whether the issue is in the CNG-specific hardware or in shared engine components.

  4. A CNG high-pressure regulator output is measured at 120 psi when specification is 90–100 psi. This condition will MOST likely cause:

    Answer: Rich running condition and potential injector or seal damage over time

    Excessive delivery pressure forces more fuel than commanded into the engine, causing rich operation and excessive pressure on injector seals and downstream components.

  5. A CNG fueling receptacle is found to be damaged with a cracked check valve housing. The technician should:

    Answer: Tag the vehicle out of service and replace the fueling receptacle assembly before refueling

    A cracked fueling receptacle housing is a structural failure that risks fuel release during the high-pressure fill process and must be replaced before any refueling.

  6. When using a manometer to check CNG low-pressure regulator output, the reading fluctuates widely with engine speed changes. This indicates:

    Answer: The regulator diaphragm is damaged or the spring is fatigued, causing inconsistent pressure regulation

    A healthy regulator maintains steady output pressure across engine speed ranges; wide fluctuations indicate a diaphragm or spring failure inside the regulator.

  7. A CNG vehicle storing a P0093 (fuel system leak detected — large leak) DTC should be diagnosed by:

    Answer: Performing a static pressure decay test on the fuel system to locate the leak source before any repair

    A static pressure decay test holds the system at pressure and monitors for a drop, identifying a real large leak versus a sensor or ECM fault before any parts are replaced.