if you unplug any sensor on the 170s and then plug them back in the E M L will come on every time as the ecu thinks there is a fault ( disconnect the battery and leave it for 15 mins then connect it back up and the light will go off and the ecu will reset it's self) as for the flat spot try cleaning out the throttle body as they get caked up over the years and hence a flat spot and rough idle done mine a few months ago and been fine since