Basic troubleshooting would include first making sure your wiring is good, check the sensor end to computer end for continuity, it should be less than 0.7 ohms assuming 10' or less wire run. Look for chafes/frays/etc.
If wiring is fine, then next likely culprit is the MP sensor itself. I'm not 100% familiar with which sensor the GRT uses, but typically MP sensors are absolute pressure sensors which usually means they have an internal vacuum reference. If that reference has failed, then the sensor is telling you presure relative to ambient so it's essentially become a differential pressure sensor which is useless for measuring MP.
Reggie