Abstract: A contamination sensor for monitoring exhaust path of the cleaning apparatus of an electrophotographic copier/duplicator to determine a malfunction condition in the cleaning apparatus. The sensor includes a light emitting diode (LED) placed in the exhaust path of the cleaning apparatus downstream of the final filter thereof and an optical sensor positioned to view the LED. The optical sensor produces a signal dependent upon the concentration of particulate toner in the exhaust path. The optical sensor is connected to the inverting input of a integrated circuit comparator wherein the signal from the optical sensor is compared to a reference signal. When the reference signal is exceeded, a circuit breaker is opened to give an appropriate alarm and/or shut down the copier/duplicator.