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 shows a fault code for high fuel rail pressure. What is the FIRST diagnostic step?

    Answer: Verify actual rail pressure with a calibrated gauge against the ECM reading

    Verifying actual pressure with a calibrated gauge confirms whether the fault is a real pressure issue or a sensor/ECM reporting error.

  2. Which symptom most strongly suggests a failed CNG fuel temperature sensor?

    Answer: Erratic fuel trims and rough idle varying with ambient temperature

    The fuel temperature sensor affects fuel density calculations; a faulty sensor causes fuel trim errors that change with temperature swings.

  3. When performing a CNG injector balance test, an injector shows significantly lower flow than the others. The BEST next step is to:

    Answer: Inspect and clean or replace the suspect injector

    A low-flow injector indicates restriction or failure, and it must be inspected, cleaned, or replaced to restore proper fueling balance.

  4. A CNG vehicle exhibits hard starting in cold weather but runs normally once warm. The most likely cause is:

    Answer: A malfunctioning coolant-heated low-pressure regulator

    CNG low-pressure regulators rely on engine coolant heat to vaporize and regulate gas; a malfunctioning heater circuit causes cold-start difficulties.

  5. During a CNG system leak test using electronic gas detection equipment, the technician should move the probe at a rate of approximately:

    Answer: 1–2 inches per second

    Moving the probe at 1–2 inches per second ensures the sensor has enough dwell time to detect low-concentration leaks accurately.

  6. A fault code indicates the CNG solenoid shutoff valve is open-circuit. Technician A says to check for a broken wire at the valve connector. Technician B says to replace the ECM. Who is correct?

    Answer: Technician A only

    An open-circuit code most commonly results from a wiring fault such as a broken wire, connector corrosion, or a failed solenoid coil — not an ECM failure.

  7. What is the correct procedure before opening any high-pressure CNG line for service?

    Answer: Purge the line with nitrogen after closing the manual valve and venting residual pressure

    Closing the manual valve, venting residual pressure, and purging with nitrogen ensures the line is safe to open without releasing flammable gas.