mmm, interesting. I had a bit of a read of the engine control system philosophy in the workshop manual. According to the manual, the boost sensor does nothing in regard to engine control, that is it is only used for high and low pressure detection and throws an error code if detected, it does not actually dictate how much fuel is added by the fuel pump. In 'Normal' control (ie not starting or idling) the only inputs the ECM looks at is Crankshaft Position Sensor, Accelerator Position Sensor and Vehicle Speed Sensor. My boost sensor did not throw an error code (only when unplugged) in normal operation. The boost sensor gives a linear proportional voltage output ranging from 0.5V @ 13.3kPa and 5V @ 253.3kPa (1.9psi to 36.7psi). I am going to check my old one for the output signal, but as I said, it did not bring up an error code with the scan tool the diesel place had, it did however when we unplugged it. The power increase after changing it was substantial, but I have no figures to back this up with, but I can say I have been very close to selling the car because of the low power, but now am completely happy with it. I will keep digging in the manual but if anyone has any other thoughts on what the boost sensor actually does i would like to know.