What should be done when CNG is being transferred to a motor vehicle?

When transferring compressed natural gas (CNG) to a motor vehicle, it is essential to turn off the engine. This is crucial for several safety reasons. An operating engine could ignite stray gas that might escape during the transfer process or create sparks that could lead to fire or explosion hazards. By ensuring the engine is off, the risk of accidents is significantly minimized.

Additionally, turning off the engine helps maintain a stable environment during the fueling process, creating a safer atmosphere for personnel involved in refueling and preventing any unintended emissions from the vehicle’s exhaust. This practice aligns with safety protocols that are in place to protect both the individuals handling the CNG and the vehicle itself.

The other options do not ensure safe handling of CNG during transfer, as having the engine running or the ignition on could lead to ignition sources that might ignite gas if any leaks occur.