rpm sensor?

I have a 2001 Vauxhall Vectra 2.0 DTi, recently I noticed the turbo didn't seem to be kicking in and thought it a bit strange, the EML turns on and off irregularly but normally around 1800 revs around the time the turbo normally kicks in. I have had it attached to a computer and the error code it comes up with is 'RPM Sensor'. The car continuously lacks power whether the EML is on or off but more so when it is lit. Having trawled the internet for weeks trying to find a solution I have found nothing and so am hoping that someone here can help me, thanks in advance.