If your car's discharge gas sensor isn't working, you may notice symptoms such as poor fuel economy, rough idling, or a check engine light. You can diagnose a defective discharge gas sensor using a diagnostic tool, such as an oscilloscope, to check the sensor signal. You can also test the sensor heater using a multimeter to ensure that it's getting power from the car battery. If the sensor is functioning properly but the lambda value is low, there may be a problem with a second sensor, such as a band sensor, that's causing a low oxygen reading.