Abstract Using proper oils to lubricate an engine is very important. There is a need for a sensor to monitor the condition of the oil to ensure the engine is properly protected. In this paper, a microsensor is put in a test engine which is operated at the ASTM Sequence IIIE conditions. The sensor has two electrodes with an interdigitated pattern. The spacing between electrodes is about 5μm and the sensing electrodes cover 7 mm×7 mm in area. With an a.c. sawtooth waveform voltage applied to the electrodes, an oil-condition-dependent current can be collected by the sensor. This current is rectified and converted to a d.c. voltage output. Different brands of engine oils are used in the test. Results show that the sensor has a voltage output that increases with the oil degradation time. The sensor is able to detect glycol contamination in oil as well.