I recently purchased a 2003 Expedition Eddie Bauer with a 5.4L engine (2WD). The previous owner (my father) told me of an issue he had with the back-up alarm sensors. When the vehicle is put in reverse after being parked in the garage or in the shade for a significant amount of time, the back-up alarm is silent if no obstruction is in the way (as it should be). However, once the vehicle is out in the sun, any time the transmission is put in reverse, the alarm sounds, with or without an obstruction.

I've cleaned the "eyes" in the bumper, but this did not help. The local dealer didn't offer any suggested fixes. Anybody out there have some ideas on this?

 

Did you clean all 3 eyes? There is a radar sensor in the center, and two ultrasonic sensors onthe right and the left. Here's some stuff I copied from the manual.
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Principles of Operation

NOTE: The liftgate must be closed for correct operation of the parking aid system.

The parking aid system detects objects behind the vehicle when the vehicle is in reverse gear. The parking aid module (PAM) does this by calculating the distance to an object by the use of two ultrasonic sensors and one radar sensor mounted in the rear bumper. The ultrasonic field is emitted in a semicircular area around the rear of the vehicle. The parking aid sensors can detect objects approximately 5 m (16.4 ft) when the vehicle is moving and 3 m (9.8 ft) when the vehicle is stationary to the rear of the vehicle, 50 cm (1.6 ft) to the rear side of the vehicle, and 16 cm (6 in) above ground. A 750 Hz variable warning tone is generated from a parking aid speaker attached to the PAM. The parking aid speaker increases its warning tone rate as the vehicle gets closer to an obstacle. When an object is detected within 45 cm (18 inches) of the sensors, the warning tone becomes continuous.

The parking aid system is enabled when the vehicle ignition is in RUN and reverse (R) gear is selected. The system can be disabled by pressing the menu button on the message center. The parking aid system will indicate it is disabled by illuminating the park assist off located in the message center. The system will always enable when the ignition is transitioned to RUN, or upon entry into REVERSE (R) gear is selected.

The parking aid system will be disabled if a fault is detected in one of the three sensors, the parking aid speaker or the PAM. This will be indicated by the parking assist off in the message center. The PAM is on the International Standards Organization (ISO) 9141 network and can be diagnosed with a diagnostic tool.

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Continuous or intermittent tone when no obstacles or fault codes are present (certain obstacles may appear "stealthy" to the system, depending on geometric shape, size and material).
Dirty or iced over parking aid sensor(s).
CLEAN the rear bumper and sensors with high-pressure water.

Parking aid sensor bezel(s) or parking aid sensor(s) locked into the rear bumper incorrectly.
INSTALL the parking aid sensor bezel(s) into the rear bumper correctly. INSTALL the parking aid sensor(s) into the parking aid sensor bezel(s) correctly.

Parking aid sensors are not aligned correctly.
CARRY OUT the azimuth check. REFER to Azimuth System Check Minimum Detectable Object Verification in this section.

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NOTE: The detectable object used in this system check can be fabricated using a 7.62 cm (3 inches) diameter plastic pipe, 99.06 cm (39 inches) in length (available as PVC pipe, or similar from a hardware or plumbing supply).

NOTE: The following system check should be carried out with the vehicle on a level surface.

NOTE: The parking aid module (PAM) system will default to on when the ignition key is cycled from the OFF position to the ON position.

Turn the ignition to the ON position, engine off.
Set the parking brake on.
Place the gearshift in REVERSE (R).
Verify that the PAM detects the detectable object when it is placed within the five specified locations (P1, P2, P3, P4 and P5). These zones are shown ideally and are not absolute. Pipe should be inside of locations.

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Elevation System Check
Check the parking aid system to make sure that the system does not see signals caused by ground reflections.

The following system check should be carried out with the vehicle on a 4.57 m (15 feet) wide by 3.05 m (10 feet) deep smooth concrete surface, free of all obstacles. The area should also be free of noise from fans and pneumatic tools.
Turn the ignition to the ON position, engine off.
Set the parking brake on.
Place the gearshift in REVERSE (R).
NOTE: No audible warning tones should be heard. If audible warning tones are heard, check to make sure the bumper is installed correctly and not tilted downward so that the sensor is pointing towards the ground.

Verify that no audible warning tones are heard.
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I am wondering if the system is detecting the ground. If you are in the garage, most driveways tilt downward away from the door so rain will not run under the garage door. They are saying any little fender bender may get the sensors out of wack and start reading things they should not.