How can I determine whether it is the sensor or the controller that is not working properly?

For mechanical meters, a voltmeter can typically be used to check for pulse output. See this video for how this should be done. For the insertion magmeter, the pulse output is too fast to be seen with a voltmeter, this is why there is an LED indicator light which indicates whether the meter is sensing flow. The “Tap Test” can be used to determine whether the controller is working properly.