How to access rear park assist sensor that is pushed into the bumper? do i go in from the trunk or under the bumper?

Parking Aid Navigator The parking aid system consists of the following:

• one radar sensor (located in the center of the rear bumper)
• two ultrasonic parking aid sensors (located on the left and right side of the rear bumper)
• parking aid module (located behind the LH C-pillar trim panel)
• parking aid speaker-audible warning tone (located in the parking aid module)

Expedition The parking aid system consists of the following:

• four ultrasonic parking aid sensors (located in the rear bumper)
• parking aid module (located behind the LH C-pillar trim panel)
• parking aid speaker-audible warning tone (located on the parking aid module)
• reverse gear select input (to the parking aid module from the reverse lamp circuit)
• The parking aid system is enabled when the ignition switch is in the RUN position and REVERSE is selected. The system can be disabled through the message center RESET switch located in the instrument cluster. The parking aid menu is displayed until the transmission is taken out of REVERSE. While the menu is displayed, pressing the message center RESET switch changes the parking aid system status to OFF.

See the awd fuse in under hood fuse box, not only check the fuse, use a test light and check voltage on fuse circuit, key on. There shows to be four park assist sensors at the rear bumper, they have three wire connector, that is ref voltage--ground--signal. You also have a rear park assist module, At the moment, I don't know where that is located? Any applicable trouble codes relating to rear park assist? I have some text explaining the operation, but, not really any troubleshooting sequence.

Most Citro?n models require the bumper to removed to access the parking sensors, this sometimes incurs the removal of the rear lights and inner wheel arch trims to reach screws to remove.

open bootlid (trunk) remove boot rubber and inside plastic panel on the body not bootlid there should be fasteners then trace fasteners at both fender linings and remove all fasteners under the bumper next gently pull both sides of the bumper away from the fender it only moves about 10-20mm then slide bumper off the vehicle remember as the bumper comes off you still need to disconnect number plate light and parking distance sensors if your vehicle has also if it has disconnect reverse camera

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.

1. Open liftgate.

Fig. 12

1. Remove two push pin fasteners per side at the liftgate tower area (Fig. 12).
2. Hoist and support vehicle with safety stands, as necessary.

Fig. 13

1. Disconnect the park assist wire connector under the vehicle and the push pin fasteners attaching wire harness under the vehicle (Fig. 13)
2. Remove the four push pin fasteners attaching rear fascia to reinforcement bumper.
3. Remove two screws (each side) attaching fascia to rear fascia bracket in the wheelhouse.
4. Remove the screw on each side attaching the upper fascia in the tail lamp bucket area.
5. Remove rear fascia from vehicle by lifting sides of fascia up and outboard to release from rear fascia brackets.
6. Place fascia on a clean surface.

Fig. 14

1. Remove park assist sensors, if necessary (Fig. 14).

Fig. 15

1. Remove spring clips attaching upper and lower fascia together (Fig. 15).
2. Using a trim stick release locking tabs and separate the upper and lower fascia.

Fig. 16
Fig. 17

1. Remove move the bright chrome molding from the upper fascia (Fig. 16). Using a trim stick, starting at the out edge of the fascia release the locking tabs (Fig. 17). Grip the molding and pull outward from the fascia while releasing the locking tabs.
2. Remove molding from fascia.

INSTALLATION

1. Place bright chrome molding in position on the upper fascia.

Here you go

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.

You only need to remove the bumper cover. Remove both tail lamp assemblies. Under the tail lamps you will a 10mm and a 8mm bolt, remove these. Look into each rear wheel well rearward and you will find a T20 torx screw on both sides, remove these. On the top of the rear bumper, between the bumper and the bottom of the liftgate opening you will find 6 to 8 plastic rivets that can be removed by pulling the center pins out, remove these. At this point you should be able to lift upward and rearward and release the bumper cover. Once the cover is free you must disconnect the electrical connector to the sensor wiring harness. With the bumper cover on the ground or a bench, you will see 4 9/16" nuts that hold the styrofoam bumper cushion to the bumper cover, remove these. Now you should be at the point where the backup sensors are accessable. When you install the new sensors be sure your electrical connectors click when they connect. these connectors go on hard and if they don't click you will be doing this job again. Reassemble in reverse order.
Hope this helps

The rear tail lights can be removed by unscrewing the lamp module from inside your trunk, it's a large black knob that you can push on once it's unscrewed to make the lamp module pop out of the bumper socket. The "white" tail lights are accessed from underneath the car by reaching up behind the rear bumper.

Most likely the problem you are experiencing with the park assist is a fault in one of the sensors mounted in the rear bumper. I would not consider it to be a major fix, however there will be a few hours labor involved as the bumper cover needs to be removed to replace the sensors.