Patent ID: 8577231

Claim:
An image forming apparatus comprising: a control unit to generate a control signal; and a developing unit comprising a toner/carrier compartment having toner and carrier therein, a toner/carrier ratio sensor to receive the control signal and to generate an output signal corresponding to a toner/carrier ratio in the toner/carrier compartment and an agitator to agitate the toner in the developing unit, wherein the control unit determines a value of the control signal which causes the toner/carrier ratio sensor of the developing unit to generate the output signal which is within a target value range, and wherein the control unit determines whether the agitator is normal or abnormal based on a detected periodic fluctuation of an output voltage of the toner/carrier ratio sensor, where the agitator is abnormal when a predetermined voltage is not periodically detected with the detected periodic fluctuation of the output voltage of the toner/carrier ratio sensor.