Code 25 if no effect on gas mileage what is the problem and why is code always showing up?

25 - automatic idle speed motor driver circuit, short or open detected

Typically this is due to a break in one of the four wires going to the idle speed motor. Could also be a bad connection in its plug. It is rare for the motor itself to cause the problem. You should get a nice "idle flare-up" to 1500 rpm when you start the engine. If you do, the motor is working and the code is being set due to an intermittent connection or one of the wires is rubbed through and grounding out to the body.

You must have a very early production van. '96 and newer models have "OBD2" emissions systems, (on-board diagnostics, version 2). Those use three-digit fault codes that show up in the odometer display.