99 Ford WindStar Door Ajar/Interior lights won't turn off

Symptoms:

Door ajar and interior lights always on even when all the doors are closed on this 1999 Ford Windstar. There is no check engine light coming in the dash.

Basic Concept:

Door latch switches are monitored by the vehicle's body computer called GEM. If one of them is stuck or defective, this problem will occur. Normally, a garage can use a scanner to monitor the GEM functions so that when you open the door, a code will appear to show the defective switch. In this vehicle, the check engine light computer is a separate system and does not control the body function such as the side doors and rear door openings.

Repair Strategies:

Since a generic scanner cannot be used to monitor GEM functions, try lubricating all the door latch switches with WD40 penetrating oil. Soaked them for about 10-20 minutes and try slamming the doors a few times to free up the switches. Usually, this method should fix the lights but if not, a garage scanner can be used to scan the GEM and replace the bad switch.