I was thinking about something to suggest for you to try at home, and the best thing to do would be to focus on the most vulnerable part of the system. The sensors in the bumper and the wiring to them are the most likely parts to develop problems, simply due to the exposure to water splashing and debris.
I would suggest unplugging each sensor, and cleaning the terminals of the plug and sensor, then reconnect after using some dielectric grease to protect the terminals from moisture. While doing this, you can look for damage to the harness that goes to the sensors, and possibly find a broken wire or other obvious fault. This requires removal of the bumper cover though, to access the sensors. Some of the harness should be accessible where the body harness connects to the bumper harness, and sometimes a bad connection there will cause problems. It might help to disconnect, clean and use dielectric grease on the harness connector.
The sensors themselves are pretty difficult/impossible to diagnose without a scan tool. If you find one that is obviously damaged or affected by water, it may be worth replacing and retesting. You may end up at least needing to get the codes from the PAM scanned, and then post back, I can give the pinpoint tests for any of the codes you may have. Also, if you want the wiring diagrams for the system, let me know.
Ford did issue some Technical Service Bulletins for this system. One of them says that you should replace the module if you have code C1983:
|LINCOLN:||2003-2005 Town Car|
2003-2006 LS, Navigator
This article supersedes TSB 06-20-13 to update the model year coverage and Service Procedure.
Some 2003 Expeditions built 4/1/2002-1/31/2003, 2003-2005 Town Car and 2003-2006 Lincoln LS, and Navigator vehicles may exhibit an inoperative parking aid system and diagnostic trouble code (DTC) C1983 stored in memory of the Parking Aid Module (PAM).
Follow the Service Procedure steps to verify the vehicle has DTC C1983 in memory. If DTC C1983 is the only DTC present, it may be necessary to replace the PAM.
|NOTE:||IF THE VEHICLE ALREADY HAS THE LATEST LEVEL PAM MODULE (REFERENCING THE PART NUMBERS LISTED IN THIS TSB) AND C1983 IS THE ONLY DTC PRESENT, CONTINUE WITH NORMAL DIAGNOSTICS IN THE APPROPRIATE WORKSHOP MANUAL.|
- Use the IDS to run PAM self-test.
- If DTC C1983 is the only code present, replace the PAM referring to the physical replacement procedure in the on-line Workshop Manual (WSM). If other codes are present, repair other DTCs before continuing with this TSB.
Refer to the following for the correct WSM section:
- Expedition/Navigator/Town Car: Section 413-13
- Lincoln LS: Section 413-00
- Reprogram the PAM using the following procedure:
- Start vehicle and place transmission gear selector into Reverse (R), wait a minimum of 10 seconds.
- Shift the gear selector into Drive (D), drive the vehicle ABOVE 25 MPH (40 Km/h) for a minimum of 15 seconds.
- Stop the vehicle and cycle the key off, then back on.
- Repeat Steps a, b, and c, three (3) more times for a total of four (4) times.
|NOTE:||THE IDS NOW HAS THE CAPABILITY TO CHECK THE PIDS FOR THE RADAR SENSORS. USE THESE PIDS TO VERIFY OPERATION OF THE TWO (2) RADAR SENSORS.|
|PART NUMBER||PART NAME|
|4L7Z-15K866-AB||PAM - 2003 Expedition built 4/1/2002-1/31/2003|
|4L7Z-15K866-AB||PAM - 2003-2004 Navigator|
|5L7Z-15K866-B||PAM - 2005 Navigator built through 12/9/2004|
|6L7Z-15K866-AA||PAM - 2005-2006 Navigator built on/after 12/10/2004|
|3W1Z-15K866-AC||PAM - 2003 Town Car|
|4W1Z-15K866-AB||PAM - 2004-2005 Town Car|
|6W4Z-15K866-A||PAM - 2003-2006 Lincoln LS|
Another TSB is for false activation of the chime (which I know is not happening here, but there may be useful info): TSB 05-6-4
The difficulty is, that the bumper cover must be removed to get to the sensors. If you want I can post instructions. And if the module ends up being bad, all the effort of removing the bumper is wasted.
Some people have had luck with replacing all 4 sensors at once (even though usually not all fail at once). This approach only makes sense if you get a killer deal on the sensors, from Ford they start at around $80 or more. There was a seller on eBay selling surplus parts for $8 each, that is why it made sense for them. Unfortunately, I see no eBay deals like that at the moment.
I think there is a possibility of a bad module, like the TSB for C1983 says, or one or more bad sensors. These are the most likely faults.I suppose you could buy one working sensor either used or new, and try installing it in place of one sensor at a time, and you might find the bad sensor this way. When the bad sensor was replaced, the system would work. Drawback, though, I believe you have a different part number for inner (by the license plate) and outer (by the end of the bumper) sensors. Some of the Ford shop manual diagnostics with the scan tool use a similar method, telling the technician to swap a bad sensor to a different spot to see if the code follows the sensor or not. If the code follows the sensor, the sensor is bad. If not, then the wiring to the sensor's original position is the fault.