you should have an infrared LED and a phototransistor, either across from each other, or next to each other. if they are across from each other, the motor starts when the transistor sees no IR from the LED. if they are next to each other, the motor starts when the IR from the LED gets reflected off of a piece of paper. in the first case, the transistor is conducting while IR from the LED is hitting it. when the beam is broken, the transistor goes open circuit, and a logic circuit inverts this signal and applies power to the motor. if they are next to each other, the transistor is open circuit until the beam reflects off of a piece of paper, the transistor turns on, the signal goes through a buffer circuit that applies power to the motor. if the LED and sensor are across from each other, it's possible the transistor is shorted or leaky. this will show as a low resistance when you check between the C and E of the transistor with an ohmmeter. if they are side by side, the LED could be bad, either shorted or open, or the transistor could be open (and without a base lead on it, it's very hard to test for that with a meter, except by removing it from the circuit, connecting a meter across it and holding it up to an INCANDESCENT light bulb (CFL lights don't emit enough IR to turn the transistor on). you want the positive lead of the meter on the collector lead of the transistor, and the meter in diode check mode to test the transistor. IR LEDs usually have a low enough forward drop to read on the meter. in one direction it reads open, in the other direction, you will see the forward drop voltage on the meter.