Ford Mercury Villager 1997 Owner's Manual page 16

A problem with the system is indicated by one or more of the
following: the readiness light will either flash or stay lit, or it will
not light.
If any of these things happen, even intermittently, have
the air bag system serviced at your Ford or Lincoln-Mercury
dealer immediately.
Supplemental air bag readiness light
Service Engine Soon Warning Light
Your vehicle is equipped with a computer that monitors the
engine's emission control system. This system is commonly known
as the On Board Diagnostics System (OBD II). This OBD II
system protects the environment by ensuring that your vehicle
continues to meet government emission standards. The OBD II
system also assists the service technician in properly servicing
your vehicle.
The Service Engine Soon indicator light illuminates when the
ignition is first turned to the ON position to check the bulb. If it
comes on after the engine is started, one of the engine's
emission control systems may be malfunctioning. The light may
illuminate without a driveability concern being noted. The
vehicle will usually be driveable and will not require towing.
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WARNING