What could be the main reason my service engine soon light would just come on in my 1999 Ford Escort?

Question

The car sounds great. I checked the oil,transmission, and antifreeze fluids. They are fine. When I bought the car last month the man said it needs a oil changs at 105,000. It is only now 103,979.

Answer 1

There are SEVERAL reasons the "service engine soon" light could be on.

To pin the problem to a particular reason, would be impossible without reading the trouble code(s) stored in the engine computer with a scanner.

If you have an Auto-Zone parts store near you, they will read the code(s)  for free  and give you a print-out of the stored trouble code(s). I am going out on a  limb here, but this is the "most common" Ford sensor failures I have experienced:

1. EGR(Exhaust Gas Recirculating) Pressure Differential Valve defective

    (EGR circuit voltage too high, too low or circuit open)

2. ISC(Idle Speed Controller) sludged or motor burned out.

    (Idle speed above/below desired idle)

3. Front/Rear Oxygen Sensor defective.

This is not to say you have a bad sensor, it could be something as simple as a broken/leaking vacuum line.  

Answer 2

some garages have a computer that they can hook up to your car. Your car sends a code to the computer that tells it why the light came on. Chances are it is an oxygen sensor. The garage can reset the car so that the light goes out, but if you don't fix whatever made it come on in the first place it will just come back on.