Here are the testing instructions

1. Pull black from terminal 7 so we are isolating the sensor
2. Hold one lead of the ohm meter on the black wire
3. Put the other lead on the blue (8).  This should be 2Mohms when sensor is dry and 10-300Kohms when wet
4. Put the other lead on red (6).  This should be 50 ohms
5. Put the other lead on Brn/Slab (10).  Check the resistance chart in the manual.  This should correspond to slab temp
6. Put the other lead on Yel (9).  Check the resistance chart in the manual.  This should be close to slab temp

If any of these are out of spec than you may have a bad slab sensor

The error can only happen in one of 3 places.

1. Sensor itself
2. Wiring like a bad splice, staple in wire etc
3. Control itself

The only way to know is to isolate sensor and start measuring.