SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Sorry, I originally posted this in the wrong topic forum.

I have been experiencing a really strange problem with the Hunter Mini Clik rain sensor.
It seems that everytime it rains heavily, the sensor, which is completely filled with water, is not registrating on the Pro-C controller- "Off-Sensor". Controller is located in the basement.
After a few days of dry weather, upon testing it, it registers at that point. Eventually however, the sensor fails to work.
The contractor has come out numerous times and has replaced the sensor three times already; apparently it shorts out after awhile.
The sensor is wired underground with two splices-one behind the siding of the house(wire then gets buried and runs underground) and another splice in the valve box. Thsi wire then gets buried, comes up and enters the basement thru the same hole where the water pipe enters the house. The splices are made with waterproof wire nuts.
I am now thinking the problem has to be with the underground wiring getting wet. Upon closer examination,the first splice is made with two strand wire, but I am not sure this wire is rated for direct burial. The second splice/wire from the valve box into the ground is done with what seems to be 18/2 thermostat wire. Quite a bit of water collects in this vicinity when it rains.
I am thinking of replacing the sensor and running the wire directly thru the siding into the basement with the 25 ft of 20 gauge wiring that comes with the sensor. of course this would require a hole to be drilled thru the siding, but at this point anything is better than burying the wire again.
I am assuimg the 20 gauge wire that comes with the sensor is indeed suitable for wet locations?
Anyhow, any thoughts, suggestions on my frustrating predicament?
Thank you.