For what it’s worth, I didn’t mean take the sensor out of the wall, but just electrically unplug it from the controller to see what it does on its own when you turn on the water.

Yeah I figured that but the terminals on the sensor are hard to reach so I was figuring I would need to remove it. But then it occurred to me that I could leave the thing in place and do the isolated test by unplugging the X2 connector from the motherboard and easily access the pins through that connector. So that’s what I did. Results:

• at rest, the signal wire is 4.75 V
• water running, the signal wire is 2.3 V

So in isolation the sensor worked correctly. Then I plugged it back into the motherboard and retested to confirm again the bad voltages. But in fact the readings were correct. It’s unclear why it works now. I wonder if the unplugging and replugging of the x2 connector improved a connection that deteriorated somehow.

Thanks for saving me €36! However incidental. If I had not done the test in isolation, I probably would not have messed with the X2 connector. I would have normally just replaced the sensor as an experiment.