The crash abosorbing bumper shocks can fatigue and/or break in a good crash. They're designed for one time use and should both be replaced in the event of a hard hit. I had one get broken in a rear ender that didn't cause any other damage. The question is how hard a hit can they take before they need to be replaced. My rule of thumb is to pull out hard then push in hard on both sides of the bumper. If you get any movement out of the bumper then that shock needs to be replaced. Next, compress the bumper by putting the car in gear and press the bumper into a solid concrete wall using a block of wood for protection then let it relax. If the bumper doesn't spring back out evenly on both sides then at least one of the shocks will need to be replaced. Now go underneath and mark the exposed shock struts in their relaxed position after being pushed in. Now pull out hard on the bumper and if either line moves then that shock should be replaced.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now