It is not safe to leave a car supported solely by a jack for any extended period. A jack is designed for lifting a vehicle, not for holding it up. For any work that requires you to be under the car, or for storage beyond a few minutes, you must use jack stands. Leaving a car on a jack risks catastrophic failure of the lifting mechanism, which can lead to the car falling and causing serious injury or death.
The primary risk is that hydraulic jacks can leak fluid or slowly lose pressure over time, while mechanical scissor jacks are unstable and can easily collapse if bumped. A car's weight is immense, and a failure is sudden and irreversible. For safety, the standard practice is to lift the car with the jack, then immediately place it on a set of jack stands rated for the vehicle's weight. The jack can then be slightly lowered so the stands bear the full load, providing a secure and stable support system.
Here is a comparison of common lifting tools and their purpose:
| Tool | Primary Function | Safe for Prolonged Support? | Key Consideration |
|---|
| Hydraulic Floor Jack | Lifting the vehicle quickly. | No | Can slowly leak down; not a storage device. |
| Scissor Jack | Emergency tire changes. | No | Highly unstable; for roadside use only. |
| Jack Stands | Securely holding the vehicle. | Yes | Must be rated for the car's weight and used on solid, level ground. |
| Ramps | Driving onto for front-end work. | Yes | Excellent for wheel-on services like oil changes. |
If you must leave the car elevated for a day or two (e.g., waiting for a part), using four jack stands at the vehicle's designated lift points is the only moderately acceptable method. Ensure the parking brake is engaged, the transmission is in "Park" (or gear for a manual), and chock the wheels on the ground. However, even this is riskier than having the car on its wheels and should be minimized. The golden rule is never to trust your life to a jack.
