Originally Posted by
sierrafery
That reading is a boost calculation made by the ECU by extracting the AAP from the MAP reading, you need to read the manifold pressure in kPa from "inputs fuelling" to see the MAP sensor's direct output, there is sucction in the inlet manifold with engine running so if the boost is low or there is a resriction on the line a negative boost reading doesnt necessarily mean that the MAP sensor is fubar. Remove the sensor to be in the air, let it plugged in and see the kPa output, if it's equal with the real ambient pressure shown by a barometer that negative reading is not a sensor issue. However if it's old it's not a bad move to fit a new one to be 100% ruled out just dont buy some cheap aftermarket, go for Bosch if you dont want to spend on a genuine LR which would be the best.
if you clean the sensor you must spray well iinside the bottom of it as that small hole near the probe to be free of muck cos there's the pressure sensor, shown with the arrow, the probe is for IAT