calculate input resistance with known adc value
The voltage at the output of the mux is \$\frac{5.0R_X}{R_X+12.4K +Rds(on)}\$, and we can ignore Rds(on) since it's typically a couple tenths of an ohm. 

The voltage at the input to the ADC is \$\frac{5.0\cdot 0.4\cdot R_X}{R_X+12.4K +Rds(on)}\$ because of the divider resistors R149/R150.

Solve for Rx. Then subtract the resistance of the common mode filter and wires (or ignore it) to get the sensor resistance.