1995 disco code 48

My disco is throwing a code 48 which means stepper motor per the fault codes. The engine is idleing really high. The tamper proof screw to set the idle setting has not been messed with. I removed the tamper proof plug only to find that the alllen head bolt to set idle setting is screwed all the way down and if screwed out only makes the idle higher and worse. HELP! Engine is running to strong and idleing really high to where it kinda makes me paranoid to crank. How do I know if stepper motor is bad???