QUESTION: I am having a problem with my 2002 Ford Explorer anti-theft system going off by itself all the time. Do you have any recommendations on how to fix this?

—Jeff Sanchez

ANSWER: Let’s see if we can get you out of the doghouse with your neighbors. Your Explorer has two anti-theft systems. The passive anti-theft system, or PATS, uses a transponder ignition key to deny engine function if the appropriate key isn’t used, and there’s also a perimeter anti-theft system which monitors door, hood and back hatch position while parked. Your symptom indicates a fault in the perimeter system.

In a follow-up message, you indicated the antitheft system arms and disarms properly. This can be done by pressing a door-lock button upon departure and using the key to open the door from the outside. The remote key fob and door keypad, if you have one, can also arm and disarm the system. The reason I mentioned these is it’s always a good idea to begin diagnosis by knowing what does work properly, and successful arming indicates there isn’t a continuously occurring fault.

The recommended diagnostic approach for an anti theft system fault is to connect a manufacturer specified or competent aftermarket scan tool to the vehicle and check for successful module to module communication and body system diagnostic trouble codes. The Explorer’s on-board diagnostics are really smart, with monitoring of all the perimeter system inputs and some outputs for rationality. My concern is that you have an intermittent problem while parked and the diagnostics are oriented towards continual irrational inputs with key-on, so this check may prove unfruitful.

The most likely cause of a false alarm condition is a flaky door, hood, hatch or hatch glass ajar switch. These are normally closed grounding switches, which open a circuit when a particular door, or the hood or hatch is ajar. I believe it’s designed this way to detect tampering, but the downside is a connection fault — the most common of all electrical problems — in any of the switch circuits or a dirty switch that results in an ajar message being delivered to the control unit.

Let’s try to tease the system into a false alarm. While armed, try applying various applications of force — pushing and pulling continuously, then wiggling — to each door, the hood, hatch, and hatch glass. With luck, you may trigger the alarm, indicating which of the switches is the culprit. If this doesn’t work, try doing this again under similar conditions to other false alarms, in terms of ambient temperature, humidity, etc.

As a last resort, I looked for a way to disable the perimeter anti-theft system, but it’s firmly integrated with other desired body functions you won’t want to do without. Another desperation ploy is to access and ground each ajar switch connection, one by one, checking to see when the fault no longer occurs. Rather than simple pin switches, the Explorer’s ajar switches are piggybacked atop each latch, making them tougher to get to. Begin with the hatch and glass switches.