Code 31 basically means the ECU isn't measuring a voltage between the PIM or VC terminal and E2 sensor ground.
So either the PIM or VC signal is shorting to ground, the PIM or VC signal circuit is open (i.e. a break in the wire or bad connection), there is a break in the E2 ground connection between the MAP sensor and ECU, the MAP sensor is dead or the ECU has an internal fault.
I would check the MAP sensor connector first to make sure it's good, then as Bob has suggested, if you've got a FCD fitted or any other component that taps into the MAP sensor circuit then check those connections also. If you don't find a problem and you have a spare MAP sensor swap it over and try again. Failing that, break out the multimeter and start checking the continuity of the wiring between the MAP sensor and ECU.