Check backup aid

First you need to read the code's to find out what the problem is , you don't just guess and replace part's . You really should take it to a qualified repair shop if you don't know that .
Parking Aid The parking aid system consists of the:

• parking aid sensors
• parking aid speaker
• parking aid module

The parking aid system is activated when the parking aid module receives a network messages from the instrument cluster indicating that the vehicle is in reverse. The parking aid system sounds a warning to alert the driver of stationary objects near the rear bumper when REVERSE is selected. The parking aid system assists the driver in detecting certain objects while the vehicle moves in reverse at speeds less than 4.8 km/h (3 mph). The parking aid system may not detect angular or moving objects.

Inspection and Verification

1. Verify the customer concern.

1. Visually inspect the following for obvious signs of electrical damage.
   
   Visual Inspection Chart Electrical
   • Smart junction box (SJB) fuse 18 (10A)
   • Circuitry
   • Connector(s)
   • Parking aid sensors
   • Parking aid speaker
   • Parking aid module

1. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.

1. If the cause is not visually evident, connect the diagnostic tool to the data link connector (DLC) and select the vehicle to be tested from the diagnostic tool menu. If the diagnostic tool does not communicate with the vehicle:
   • check that the program card is correctly installed.
   • check the connections to the vehicle.
   • check the switch position.

1. If the diagnostic tool still does not communicate with the vehicle, refer to the diagnostic tool operating manual.

1. Carry out the diagnostic tool data link test. If the diagnostic tool responds with:
   • CAN circuit fault; all electronic control units no response/not equipped, refer to Section 418-00 .
   • No response/not equipped for parking aid module, Go To Pinpoint Test A .
   • System passed, retrieve and record the continuous diagnostic trouble codes (DTCs), erase the continuous DTCs and carry out self-test diagnostics for the parking aid module.

1. If the DTCs retrieved are related to the concern, go to the Instrument Cluster Diagnostic Trouble Code (DTC) Index or the Parking Aid Module Diagnostic Trouble Code (DTC) Index.

1. If no DTCs related to the concern are retrieved, GO to Symptom Chart . Instrument Cluster Diagnostic Trouble Code (DTC) Index DTC Description Source Action U0159 Lost Communication With Parking Aid Module Instrument Cluster REPAIR all other DTCs first. If the DTC is still present Go To Pinpoint Test A . All Other DTCs - Instrument Cluster REFER to Section 413-01 .
   
   
   Parking Aid Module Diagnostic Trouble Code (DTC) Index DTC Description Source Action B1299 Power Supply Sensor Circuit Short to Ground Parking Aid Module Go To Pinpoint Test C . B1342 ECU is Faulted Parking Aid Module CLEAR the DTCs. REPEAT the parking aid module self-test. If DTC B1342 is retrieved again, INSTALL a new parking aid module. REFER to Parking Aid Module in this section. CLEAR all DTCs. REPEAT the self-test. B2477 Module Configuration Failure Parking Aid Module REFER to Section 418-01 . C1699 Left Rear Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1700 Left Rear Sensor Circuit Failure or Blockage Parking Aid Module Go To Pinpoint Test C . C1701 Left Rear Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1702 Right Rear Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1703 Right Rear Sensor Circuit Failure or Blockage Parking Aid Module Go To Pinpoint Test C . C1704 Right Rear Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1705 Left Rear Center Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1706 Left Rear Center Sensor Circuit Failure Parking Aid Module Go To Pinpoint Test D . C1707 Left Rear Center Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1708 Right Rear Center Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1709 Right Rear Center Sensor Circuit Failure Parking Aid Module Go To Pinpoint Test C . C1710 Right Rear Center Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1742 Rear Sounder Circuit Failure Parking Aid Module Go To Pinpoint Test F . C1743 Rear Sounder Circuit Short to Battery Parking Aid Module Go To Pinpoint Test F . U1900 CAN Malfunction - Missing Message Parking Aid Module REPAIR all other DTCs first. REPEAT the self-test. If DTC U1900 is still present, DIAGNOSE the system for a missing transmission range signal. REFER to Section 418-00 . U2051 One or More Calibration Files Missing/Corrupt Parking Aid Module REFLASH the parking aid module. If the DTC occurs after a software reflash, REFLASH the parking aid module again. If the DTC reappears after each attempt to reflash, INSTALL a new parking aid module. REFER to Parking Aid Module in this section. CLEAR all DTCs. REPEAT the self-test. Take your vehicle to a qualified repair shop before you cause more problems .

Backup Lamps
The B/U LAMP fuse in the RH I/P fuse block supplies battery positive voltage to the switch terminal of the backup lamps relay. The DIC/RKE fuse supplies battery positive voltage to the backup lamps relay coil terminal. When you place the gear selector lever in the REVERSE position, the powertrain control module (PCM) sends a class 2 message to the body control module (BCM). When these conditions occur, the BCM energizes the reverse lamp relay control circuit, closing the switch contacts, the current flows from the reverse lamp relay switch to the backup lamps. The backup lamps are grounded at G302.

There is alot involved in turn on the back up lights .
DTC B2552
Circuit Description
The body control module (BCM) controls the backup lamp relay. The BCM grounds the backup lamp relay control circuit which energizes the backup lamp relay.

2
Install a scan tool.
Turn ON the ignition, with the engine OFF.
With a scan tool, command the backup lamp relay ON and OFF.
Does the backup lamp relay turn ON and OFF with each command?
YES - Go to Diagnostic Aids
NO - Go to Step 3

DTC B2553
The body control module (BCM) controls the backup lamp relay. The BCM grounds the backup lamps relay control circuit which energizes the backup lamp relay.
Turn the ignition ON.
Shift the gear selector lever to the REVERSE position.
Do the backup lamps turn ON?
YES - Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
NO - Go to Step 3
3
Remove the backup lamps relay.
Connect a test lamp between the battery positive supply voltage circuit of the switched side of the backup lamp relay and ground.
Does the test lamp illuminate?
Before replacing parts you may not need , take your vehicle to a qualified repair shop .

See:

Backup aid not working - Ford Expedition Forum

www.expeditionforum.com > ... > General Discussion
Apr 29, 2004 - 6 posts - ‎2 authors Join Date: Apr 2004. Location: ... Suddenly the reverse sensors stopped working. ...This may be a longshot, but if I saw a the message "Check Backup Aid", I'd look to see if a fuse had burned out. Share ... HTML code is Off.

The Rear Park Aid sounds a warning tone to warn the driver of obstacles near the rear bumper.
CHECK REAR PARK AID is displayed when the transmission is in R (Reverse) and the park aid is disabled.

You can enable it under vehicle settings

here is a service message that Ford put out,, if you check the fuses you will probably see a 20 amp fuse has blow,, try replacing that , if it blows again, you have to get it to the dealer

Have the RSS module checked for DTC'S - diagnostic trouble code's . That should always be the first step . An it is
Reverse sensing system Parking Aid The parking aid system consists of the:

• parking aid sensors
• parking aid speaker
• parking aid module

The parking aid system is activated when the parking aid module receives a network messages from the instrument cluster indicating that the vehicle is in reverse. The parking aid system sounds a warning to alert the driver of stationary objects near the rear bumper when REVERSE is selected. The parking aid system assists the driver in detecting certain objects while the vehicle moves in reverse at speeds less than 4.8 km/h (3 mph). The parking aid system may not detect angular or moving objects.

1. Visual Inspection Chart Electrical
   • Smart junction box (SJB) fuse 18 (10A)
   • Circuitry
   • Connector(s)
   • Parking aid sensors
   • Parking aid speaker
   • Parking aid module

Do you know what DTC'S are .

Parking Aid Module Diagnostic Trouble Code (DTC) Index DTC Description Source Action B1299 Power Supply Sensor Circuit Short to Ground Parking Aid Module Go To Pinpoint Test C . B1342 ECU is Faulted Parking Aid Module CLEAR the DTCs. REPEAT the parking aid module self-test. If DTC B1342 is retrieved again, INSTALL a new parking aid module. REFER to Parking Aid Module in this section. CLEAR all DTCs. REPEAT the self-test. B2477 Module Configuration Failure Parking Aid Module REFER to Section 418-01 . C1699 Left Rear Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1700 Left Rear Sensor Circuit Failure or Blockage Parking Aid Module Go To Pinpoint Test C . C1701 Left Rear Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1702 Right Rear Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1703 Right Rear Sensor Circuit Failure or Blockage Parking Aid Module Go To Pinpoint Test C . C1704 Right Rear Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1705 Left Rear Center Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1706 Left Rear Center Sensor Circuit Failure Parking Aid Module Go To Pinpoint Test D . C1707 Left Rear Center Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1708 Right Rear Center Sensor Circuit Short to Battery Parking Aid Module Go To Pinpoint Test D . C1709 Right Rear Center Sensor Circuit Failure Parking Aid Module Go To Pinpoint Test C . C1710 Right Rear Center Sensor Circuit Fault Parking Aid Module Go To Pinpoint Test E . C1742 Rear Sounder Circuit Failure Parking Aid Module Go To Pinpoint Test F . C1743 Rear Sounder Circuit Short to Battery Parking Aid Module Go To Pinpoint Test F . Your best bet , take it to a qualified repair shop . Ford

The more common failure is one of the backup sensors, testing with a scanner will indicate which sensor in the system has failed and causing the Park aid to not work. You must have this info to test the system. Please rate this answer for me THankyou RJ

Might want to check the lens for the reverse sensors to make sure they are not damaged or dirty.
Next is to check the wire from the back to the sensors, some times the connectors become corroded and if the sensors are not "seen" by the PAM ( Parking Aid Module ) the system turns on, then off right away.
The sensors remove from the back of the bumper ( they do not pull through to the outside ), and the wiring harness is fairly easy to track. Check each sensor and the connector for the sensor harness.

Either the fuse is bad, or the connection to fuse block is bad which is common, or lastly, a faulty ECM

i think the computer is telling you that it is not recognizing what gear you are in as you can only start the vehicle in park or nuetral. thats the best i can do for you sorry