what is the location of the service engine soon sensor?

There is no single "service engine soon" sensor, the indicator light is either responding to a preset condition (ie mileage for an oil change) or the OBDII system is setting a trouble code. You can have the code read at any parts store (free) or mechanics/dealership (for a fee). You can even buy your own OBD code reader (they range in price from 40 to 150). Obviously you want to get the code read and determine if you have a problem needing to be addressed. It can be anything from a loose gas cap to a severe problem. If you don't feel anything wrong when driving the car, it's likely a minor issue but still, don't put it off.

there is no sensor, the light will work on the mileage since it was last re-set