Systems and associated apparatus, methods, and computer program products, relate to a small, portable diagnostic instrument for checking the correct operation of a gas metering system that include a gas meter, an electronic corrector, and a junction there between. Some embodiments may use calibrated sensors to check the operation of a rotary or other positive displacement type gas meter or turbine meter. For example, reference sensors may be temporarily applied to monitor gas temperature and pressure, as well as the motion of a flow-responsive element (e.g., impellers, turbine, or diaphragm) in the gas meter. Measured volume signals from the gas meter system may be compared to reference volume signals determined from the reference sensors to check operation of the junction, and/or the electronic corrector. Some embodiments may detect other failure mechanisms, such as bearing-related problems in the gas meter, for example.