When your lift supports will no longer open your hatch, rear glass, trunk or hood and support it in the open position, it is time to replace them. Lift support replacement is the only remedy to the sagging lid; other remedies can create an unsafe condition.
Lift supports provide lift and assist by containing a gas charge within a cylinder. This creates a force against a piston attached to a rod that transfers the force to the lid that is being opened. The rod will move in and out of the cylinder as the lid is opened or closed. The gas charge is contained by a seal around the rod. This seal will wear over time with hundreds of cycles, and allow some of the gas charge to escape, reducing the force. At some point the force remaining is not enough to lift and support he lid.
Using a prop to support a lid with failed lift supports can create an unsafe condition. If the prop is jarred, or otherwise unintentionally removed, the lid can fall causing injury to anyone under it. This can be particularly dangerous with hatches or lift gates, which are particularly heavy, and there is a greater likelihood that children may be in the path of it falling. The only remedy for your failed lift supports is replacement. To avoid creating this unsafe condition, it is best to replace the lift support when you first notice your lid beginning to sag, or when you have to provide assistance when it is opening.
Lift supports should be replaced in pairs when there are two installed. It may seem appropriate to reuse the stronger of the pair in conjunction with just one new one, and that may allow your lid to function properly, for a while. However, the new lift support will have to compensate for the older unit, which has also worn and lost some of its force. This will decrease the life of the new one, and lead to the same sagging lid again. By replacing in pairs, you are returning the function to ‘like new’ and you will maximize the life of the new lift supports.