I have a Climatemaster Ultra classic VT series residential Veertical 2 stage Geothrmal Heat pump model VT060GSSSRTDKSH. The a side keeps locking out with code 4. # diffeant heating companys have look at it and change the thermistor #17S0030N01 out two times with the same problem. the thermister is located on the frigerant line betwwen the expansion vale and the heat exchanger. Could there be something wrong with the frigerant systerm? the temp at the thermisor is 30 deg when it locks out.