'03 Sentra rear bumper cover

About 4 months ago I bought a used '03 Sentra GXE. At the time there was a slight sag in the rear-drivers side bumper cover (about 1/4-1/2
inch lower than rest of bumper). I brought this up and the dealer said it was likely a broken/missing clip. They claimed to fix it by replacing the clip and all was well until now. The sag is back. Before completing purchase my own mechanic did top to bottom and found nothing wrong (really went over it). The bumper cover itself has no visible issues, paint is great, no dents, etc, looks perfect (minus the sag).
So quick questions...
1.) Is this common? If so what is the usual cause?
2.) Now it doesn't REALLY bother me, does leaving it me pose any future issues (ie water getting up in there and rusting, it falling off, etc).
Thanks!!
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wrote:

Bumper covers are basically pretty things that hide the hole around the the bumper(impact bar) so it can move and not damage the vehicle excessively at low speeds. They are part of an "assembly" that attaches to the sheet metal on the outside edges and "energy absorbers in the middle__under the car. Parts or the "assembly" are usually constructed from Aluminium and various types of plastic. These materials tend not to rust. Look under the car and you should see weep holes to allow splash water to drain. Rust is not your problem---The bumber cover is usually held in place over urethane plastic impact strips or logs if you will. These strips act as softeners and form return devices after a low speed collision. Around the edge of the bumper cover-in the center section of the assemby--LOOK UNDER the car again, plastic interference fit type fasteners--they look like fat nails with fins--hold the cover to the actual bumper. If the cover has a section that is sagging, then some of these "finned fasteners," usually called "plastic rivets" are out of place, or the supporting urethane strip is in need of attenion. This condition could be indicative of an improper repair. Removal and replacement of the rear bumper assembly is usually not an all day project. With luck, you will find two or three bolts or nuts on each end--where the plastic meets the quarter-panel. Then, underneath , two to four bolts per attachment to the energy absorbers. It helps to have sockets and wrenches of 10, 11, 12 13, 14, and 17 millimetersizes for all the possibilites
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found
<snip>
Wow -- now that's an answer! : )
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