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Are jack stands necessary when using a floor jack?
Jack stands should always be used in combination with jacks when working underneath a vehicle. It's generally not safe for only a car jack to hold a vehicle.
Is it safe to work under a car on jack stands?
If the surface is flat and level and the car was properly lifted and is now static, and the jacks stands were properly positioned under the car, then yes it is considered safe to work under the vehicle.
Can you jack up a car without jack stands?
Yes, using a floor jack or another type, you can jack up your car and perform various repair jobs or a tire change. But if you need to work underneath your vehicle, then jack stands are important to use for your safety.