Rear bumper back-up sensor needs to be repaired

You can jack the car up and remove plastic undercover it should give you access to sensors inn bumper

If you took it off, check the wires on the sensors. IF someone worked on it and charged for it, make them fix it. Make sure it is the right bumper and the sensors were not damaged.

You have to take bumper cover off .
Remove rear bumper fascia. Refer to Rear Bumper Fascia Replacement . Disconnect electrical from the rear object sensor. Lift the locking tabs on the housing and remove the rear object sensor.
Preliminary Procedure

1. Remove the compartment sill plate. Refer to Rear Compartment Sill Trim Plate Replacement .
2. Remove rear wheel liners. Refer to Rear Wheelhouse Panel Liner Replacement .

1
Rear Fascia Push Pin Retainers (Qty: 2)
2
Rear Fascia Nuts (Qty: 4)
Tighten
10 N?·m (89 lb in)
3
Rear Fascia Bolts (Qty: 2)
4
Rear Fascia Bolts (Qty: 6)
5
Rear Fascia
Tip
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Energy absorber and fascia are removed together. Cannot be separate on vehicle.

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Disconnect the electrical connectors.

if you have something in trailer hitch blocking bumper rear bumper dirty ,damaged rear bumper by sensor[s] or damaged /malfunction of sensor check the wiring and connections behind bumper those circular inserts are the rear sensors

these bumpers are hard to repair, most places replace the bumpers because of the problems trying to repair them. you can try a body shop that specializes in fiberglass corvettes they might have the best method.

The parts are on eBay around 100 dollars. If you do the repair then you will have to remove the whole back bumper. There is no way to do this without removing the whole bumper.

disconnect the connector from the park aid module push the connector and grommet out of the back panel remove screws from lower rear of wheel arch liners remove boot floor trim remove plastic boot closing panel trim remove both boot side trims you wil find a number of 10mm headed bolts around the boot at the sides and back remove these and get someone to help you remove the bumper liner the 2 outer sensors can normally be changed without removing the bumper liner on some models

look for a broken wire at the rear bumper all 4 couldn't have failed all at once

There are six bolts from the bumper into the rear cross member to remove, accessible from inside the boot. You'll have to remove the rubber plugs to get to two of them. After that, remove the boot lining (that's the "trunk" lining if you're a Yank) and you'll see four bolts (two on each side) that secure the bumper by screwing downwards through the metal wing into the top sides of the bumper. Now go underneath the car, and there are two bolts accessible at the rear of the bumper, accessible from beneath. Now you need to uncrew a pair of 10mm plastic nuts under the wheel arch lining/mudflap and two which are behine the rear wheels. Fold the wheel arch lining away from the bumper and there's a screw in the very front of the sides of the wing right at the point where it connects to the wheel arch. remove these (one on each side) and the bumper will slide backwards and off the body. Voila. Piece of cake. Remember to disconnect the wiring to the rear sensors and pull the multiplug through out of the hole, carefully removing the grommet.

Re-fitting is a bit trickier, so use a couple of beer crates or boxes to hold the bumper up below the rear as you line up the tabs on the air exits and slide the bumper back.

  Remove the tail lamps,remove the plastic retainers from the wheel well area. Remove the retainers below the tailgate opening and below the bumper. Lift up on the sides of the bumper to disengage the sides of the bumper. Pull it back a little and disconnect the connector for the park aid sensors if equipped. Pull back and it is off.