From the pics I can trace a bit of the circuit. Hard to see the traces.
Quad op-amp U1 is has a gain stage with trimpot, so it amplifies the sensor's output, and then a transistor switch and output to the test point, and another op-amp stage and transistor switch to the MCU. Measuring the voltages on U1 will tell what's going on I think.
On the sensor PCB, their output is AC-coupled through capacitor C2. So, little DC expected. You could try measure ACV at the sensors' output for signs of life.
I'd expect <50mV DC on their pin 4 so the ~450mV measurement seems suspect.
Where is the 6.7V rail from, I thought 12V for the relays. U3 might be a 8V voltage regulator for U1, and U7 a 5V reg for the MCU. What are the part numbers on U7, U3?
The trimpots set the sensitivity (gain), so I'd give them a whirl.