What is the science behind lie detector tests? How do they work?

These tests based on your reactions so if super nervous it say your lying even if your not just nervous. I personally dont trust them

its a system that records your blood circulation and nerve sense so when you're asked a question and if you are lying your blood circulation will have a different flow which the lie detector machine (the system) will realize and beep to let others know

There are two ways of dealing with this:

1. Leave it alone, checking the rinse aid every month.
2. Find the fault.

The fault is likely with either the level detector, which will probably mean a replacement rinse aid dispenser or with the cabling. Try disconnecting the detector cables and seeing if the light turns off (make sure it's the detector cables, not the doser). If the light turns off, the likelihood is that the detector has failed. If not, then the fault will lie elsewhere, possibly with the logic board.

I can only offer a user manual to the device. The next to last page of the manual lists contact information by country. You might see if you can get the answer from a listed source.

https://www.grainger.com/ec/pdf/GasBadge-Pro-Instruction-Manual.pdf
A prior Fixya answer offers the following:

A reading of 221 ppm carbon monoxide gas is a dangerous level that should be taken seriously. You could try a new battery and reset the detector.

I'm very interested in finding a service manual for the XRS-9430 too. If anyone knows where to find this, it would be very helpful.